As for your driver, I can guess what happened.
I was using a 10-year-old Titleist D2 10.5 degree. My spin was consistently over 3500. I eventually got fit into a 9-degree Ping G430 LST with tour black shafts, and I was crushing balls further than I ever have. This was also a surprise because the LST is a less forgiving driver. This is when my handicap was around 13-14 and had a fade/slice bias.
That combo worked well, and as I played more, my swing became more draw/hook bias. My handicap got down to around 8, but on bad days, I couldn’t launch my driver at all. Sometimes my spin would be like 1k rpm (measured on my sim), and on course, low runners or duck hooks. I eventually switched to a Qi10 (core) with a mid-launch shaft (graphite design UB), now the setup is money. Way more forgiving than the LST.
TLDR; I’m guessing that you got fit into a low spin driver because you have a severe out to in swing path, even though the Qi10 LS is not a forgiving driver. I don’t like the solution by your fitter because, IMO, it’s a bandaid. Once you get better, you might end up hating your driver because it’s not forgiving at all, lol. GL!