You can TIG right over a silver brazed sleeve, it works fine as long as you’re moderately competent with the welding.
The entire seat cluster area should be at least 1.2mm wall (you can accomplish that by sleeving something thin if you’d like) extending below the seatstay and toptube joints. Otherwise expect failure down the road, generally from the seatstays cracking the seat tube.
Once you have a lot of experience you can go down to 1.1 or even 1.0 wall if you want to geek out on weight, but the handful of grams you’ll save is IMO not worth it. Failed seat clusters are almost impossible to repair.
Your existing frame, assuming you’ve already welded it, is not fixable, but it might ride ok for a while and you’ll have plenty of warning when it does fail (IMO).
-Walt