We did a full rewrite of our site years ago going from backbone to react and TS. We just did it page by page, clean sweep on each page, all new work done on a fresh slate with the old code base being abandoned page by page but otherwise left alone until all the pages were migrated at which point we could just completely drop the old code base. We’re so much more productive now and happy working on the code base. Refactors are much easier than before as well.

I’ve worked on massive jQuery code bases before and they turn into worthless spaghetti code in no time at all. Dynamically altering html with no source of truth is the worst pattern you could have on a frontend. I have a ton of respect for what the jQuery library introduced but it was never a framework and was used in a terrible patchwork way. I wouldn’t even bother trying to save any of it. Clean split and keep that codebase tightly quarantined behind lock and key and don’t let it even touch the new code base. The spaghetti code it creates is like a virus.