I personally use the Sury repos since forever and I never had any issues with them. Ondrej Sury, the maintainer of these repos, is also one of the maintainers of PHP in the Debain project, so I’d say he’s a) trustworthy and b) he knows what he’s doing, so no issues from that side either.
So here’s my recommendation:
If you have planned the migration only because of the PHP version, I would continue to use the existing server. Update everything, by using the Sury repos, as described in my previous post, and then upgrade the system to Debian 12 after that.
If there are other reasons for the migration as well, like better, more powerful hardware, more storage, memory etc…) then my preferred method would be similar to the one without migration, i.e. update everything on the first server, and only then migrate it over to the new server.
That beeig said, there might also be ways to cut a few corners, I think I even read somewhere about updating NC to 26 without having a compatible PHP version installed. I’m not sure I would recommend trying that exact thing, but it might be worth to do some additional research in in the forums before you start. The hole “Debian / old PHP version / Upgrade to 26 topic” actually came up quiet a few times here, and there are some extensive threads about this topic…
Whether you want to start experimenting, or if you decide to go the safe route, is up to you, and I’d say this should mainly depend on how easy you can recover to a previous working state in case something goes wrong. If everything is running in VMs from which you can take snapshots, why not experiment a little bit…? 
Otherwise, I would go the safe route, and regardless of what you are doing, make sure you always have working backups!
Good luck! 