I’am switched to SOLO

But theme Solo only works on d10 and if a site is blocked on d9 why can't it update it because of the theme?

D9 I switched to Olivero. Removed mayo. Updated to d10 en installed solo. Look at multivocaal.nl

I wrote 2 articles on my website how I did the upgrade from d9 to d10 and the change Mayo to Solo vanderzwetslotenmaker.nl

The theme Solo does not have two sidebars like on my site www.gmpe.it

You can make an issue on Solo and ask if it is possible.
On my website I made a workaround and the layout now is main-left-right.