I know very little about frame building but to me this doesn’t look like this frame is going to self destruct any time soon. My vote is in the ride it and watch it camp. You can always replace the seat tube after it breaks. Not likely to cause injury. I have seen bikes that could still be ridden with both seat stays broken at the seat tube junction. Riders thought something felt funny or different but the bike was still rideable (to get home not long term). That frame is worth keeping and using!