Tailoring milestone: I just let out a suit jacket for the first time!
Bit nerve-wracking tbh; I undid the lining at the hem and turned the jacket left side out and the lining came apart at least another 2" from the stress. There was SO MUCH fraying inside and I let it out (centre back seam and side back seams) less than intended because I worried it might come apart if I left any less seam allowance (left about 4mm, or 0.15").
But the good news is, it fits! It's a 6x1 DB and while it nominally fit, it was so tight around the hips that the front pulled awfully at the sole button (so much that it came off earlier and I had to re-attach the button – fucking machine-sewn buttons never last).