To repot or not to repot JWP

[Bepo]

Hello! I'm new to this site and to bonsai so I've come here for advice.
I got this Japanese White Pine last week and I noticed that the soil is really compact. I read that JWP likes a well draining substrate and really likes oxygen for the roots so I'm planning on repotting it.
So a couple of questions:

1.Is it too late to repot?
I've read that repotting should be done in early spring. Its april already and we have had abnormally warm and sunny weather here in southern Finland so Im afraid Im late.
Some sources I've seen recommend repotting before the buds swell and some during it. I tried to get some pictures of the buds, if someone can say if its still okay to repot.
Also attached is a picture of soil above and from the side. The roots are not pot bound so no need to report because of that, but im worried the roots are not fine because of the compact soil. You can see the marks where I tried to dig in the soil with my finger.

2.If I repot what kind of substrate do you recommend?
I have 1-5mm granularity akadama and Crushed pumice 2-5mm and Im thinking of creating a mix of around 60% pumice and 40% akadama. As I have no before knowledge of substrates is this mix "airy" enough for JWP?

3. Aftercare?
Despite of the warm weather we have light frost for the whole next week so Im planning on taking the plant inside to shield it from the frost. Or is it better to wait that the cold weather passes and then repot (if to repot at all)?

This is the beginning of my bonsai journey and appreciate all the help I get

 

[Japonicus]

Despite of the warm weather we have light frost for the whole next week so Im planning on taking the plant inside to shield it from the frost.

Welcome to the sight @Bepo
Not advised to protect from general freezing temps. Possibly aftercare, after repot that is,
temporarily, though not mandate. Only to protect the newly generating roots.
Leave it on the ground if it is safe to do so, regarding pissy animals.
Full Sun is mandate, though wind protection, and moisture control is good for aftercare of repot time.
Otherwise full Sun will increase health if you get the moisture right.
Advised to use a "Boon mix" of 1:1:1 Akadama, lava rock and pumice, though there are many
varying applaudable mixes to use, the Boon mix is widely accepted for conifers.
Therefore as you're beginning in the hobby, stick with a proven mix, while you are getting settled in
and comfortable with the species.

Your buds barely exposed with new growth, and impending cold weather puts you in the sweet spot to repot.
However, if you are not familiar with when the tree was last repotted, you could hold of a year or so,
but always a Spring task. White root tips are another indicator the tree is ready for repot, but in Spring.
This (white root tips) is said to begin roughly 10 days prior to buds moving, signaling green light to go.
Easier to watch the buds to begin moving. No complete drying out, and do not keep damp.
The oxygen you refer to. This is the soil breathing when you can saturate with general ease, and dry
in due time, a wet/dry cycle needs to be occurring. Just a little bit more on the moist side during aftercare
of repots.

Last picture
Do not touch (reduce, pluck needles, pinch candles or otherwise work on) the 1st (lowest) left branch for a couple years.
Then keep buds that are strong at the branch tip while removing weaker buds while building this branch.
We remove any buds greater than 2 for a bifurcation. Keeping stronger buds on weaker branches
and weaker buds on stronger branches, and other techniques, will balance energy eventually
so all candles are pinched or shoots cut at the same time with equal vigour in all areas.

Nice looking tree. Is there a graft? Foliage looks to be a cultivar, but maybe the moss on the trunk is hiding
a nice graft point?

Adding your USDA grow zone to your profile will help others know how to respond based on your weather/season.

 

[Japonicus]

but im worried the roots are not fine because of the compact soil.

These roots look quite fine to me, and sharing desired mycorrhiza fungus. It is advisable to harvest some of this fungus
to incorporate into the new potting around the newly cut roots. It will be present if you do not remove all old soil
but reintroduction gives you a head start on moving nutrients and moisture into the tree.
Don't need a lot but maybe a couple teaspoons worth.
These roots also look like this tree could be on its own roots?
The moss should be removed carefully from the trunk. Tweezers and toothbrush helps.
Out of control moss could use some white vinegar to kill the moss applied locally and
not to the soil really. It's rather acidic.

 

[Shibui]

Despite your 'abnormally warm and sunny Spring' the buds are only just starting to move so no problem repotting now or even waiting a few weeks. I'm usually repotting pies when the candles are growing so it does not seem to hurt either early or later.
White pine is a mountain tree and will laugh at light frost, even after a repot. I would avoid indoors unless real cold is predicted.

Best soil mix is the one you are accustomed to using. While it is true that roots do need air as well as water, there's a wide range of soil mixes that can provide the essential needs. Good plant growth is much more about adapting your care to the soil mix.

I have 1-5mm granularity akadama and Crushed pumice 2-5mm and Im thinking of creating a mix of around 60% pumice and 40% akadama. As I have no before knowledge of substrates is this mix "airy" enough for JWP?

I don't use akadama but I try to remove as much fine particles from potting soil as those finer particles are what filles the spaces where air should be. Personally, I would sieve the akadama to remove the 1-3 mm grains but that may not be necessary if you can control watering better than I do.

Many growers recommend adding some old soil to the new mix to help mycorrhiza fungi re-establish. That does not hurt but I stopped doing it many years ago when I found that the mycorrhiza grew just as well from the existing tree roots and from new fungi coming into the pots.

 

[Bepo]

The moss should be removed carefully from the trunk. Tweezers and toothbrush helps.
Out of control moss could use some white vinegar to kill the moss applied locally and
not to the soil really. It's rather acidic.

Thank you for all the advice!

I started to clean out the moss and made a horrifying discovery, there is a piece of wire stuck at almost at the center of the tree!
After that I noticed there is also a piece of wire still left biting in at the top of the trunk, I can't believe I missed that before!
Is there anything to do about them at this point? The lower one has grown in so I think that is impossible to remove but what about the top one?

Despite your 'abnormally warm and sunny Spring' the buds are only just starting to move so no problem repotting now or even waiting a few weeks. I'm usually repotting pies when the candles are growing so it does not seem to hurt either early or later.
White pine is a mountain tree and will laugh at light frost, even after a repot. I would avoid indoors unless real cold is predicted.

Thank you! Im leaning on repotting because its still in nursery soil and after watering the soil stayed moist for a couple days.

 

[Shibui]

Im leaning on repotting because its still in nursery soil and after watering the soil stayed moist for a couple days.

I'm all for repotting too. I encourage all my students and customers to repot trees as soon as practicable. Having all your trees in a soil mix you know and trust makes it easier to manage watering, fertilising and general care which makes it much more likely your new trees will live and thrive.