What did your old iron actually do wrong though, was it temp control drifting or just not enough weight to really sink into the berber? Asking because berber is a whole different animal with seams, that backing fights you the whole way and I've seen guys blame the iron when the real problem was not enough heat getting down into that thick backing. If the new one fixed that bedroom bubble, then the tool mattered, but you also changed two things at once with the stretch and the iron so how do you know which one did it? Did you ever run the cheap iron on a scrap piece next to the new one and compare, or are you just going off how the finished seams feel now?

Tweaking one thing, half upfront is normal in some trades but not all. Depends on the trade honestly. I've seen plenty of guys who just do deposits instead and that works fine too.