BTW migrating nuclear WP 5.8 to CP went very smooth.
No issues whatsoever.
I had to re-order and re-save the widgets thou, somehow WP 5.8 thought it does not like my widgets and put them all in a one-sidebar box instead of the areas I did have used before.
Also there where hundreds of errors related to blocks JS in widget editor after I migrated to CP (errors relating to undefined JS objects). I cleared cache and that solved the issue. Seems like WP 5.8 wanted to hunt me down to last instance
Other than that, and now manually clearing the ugly
<!--wp block--> comments out, there where no other major issues.
I even could proceed using Toolset despite their calls to do_blocks, by adding a sorts of shim to my theme that “fakes” do_blocks (and since I did not use any blocks with Toolset either, this solution works fine and silences any fatal error of “function not defined”)
So far so good, this gives me time to develop another solution in place of Toolset and keep the site running meanwhile using CP
With this, my second site is now officially running CP