My solutions use a numbers table. You’re free to try replacing that aspect of them with an inlined version like Jeff Moden uses in his string splitter, but I found the numbers table approach faster. Granted, it’s also less portable, but that’s a trade-off I’m willing to make.
What I don’t like about either solution is that I have to re-assemble the string using XML PATH. If you’ve got another way to do that, I’m all ears. I know 2017 has STRING_AGG, but that didn’t turn out much better, and it wouldn’t be usable in other supported versions.
I agree that the best route here is just to suck it up and use CLR, but if you’re going to live in a T-SQL-only world, this is probably the best method available.