(Wait for it). Best to see a fitter. You might need a different shaft profile. Flex really only fine tunes the feel. Also, flex is only meaningful in context of a single shaft line. There is no industry standard.

TT shafts come in weights from at least 100 to 125 g shafts, in addition to at least 2 flexes for each weight. Without knowledge of your current setup, the advice would be free and worth every penny.

The other question is regaining club head speed. If you haven’t had a recent evaluation, finding a TPI Instructor to assess your current physical status may find some additional swing speed. While golf specific I find I can maintain other daily physical activities better as a result.

New drivers can gain some back, but the aging process will eventually win over any new equipment.

If what you say is accurate, why do you want to change? At 88 swing speed, that's a pretty good distance and dead straight.
Every shaft is different and their flex ratings vary quite a bit. If you're looking for something more from your drives you need to identify what you want and then have a good fitter give you advice. You can wing it on your own but that can get pretty expensive, and swing speed is only one of a half dozen numbers to look at when selecting a shaft.

a heavy regular sounds a good option, should keep the speed up but stop that left hook, weirdly i had the same thing and moved from xstiff down to stiff and actually goes further

Definitely see a fitter. Had issues with back and went to see a PT and am still under his thumb. He worked on developing my core and got my speed back up. They are working on getting my distance back up over 250. Am 77 and play Fujikura Speeder S shafts, based on fitters recommendations.

I think Don and Barry have highlighted the fact there’s probably other questions to be answered vs just shaft flex.
As a high single digit golfer, what are you missing/looking to improve. If score, where are you losing shots? Tee? Fairway? Green side? Greens?
Shaft flex could probably help with shots from the fairway into greens. But if you’re struggling on the greens, maybe just need more practice time with your putter.
Good luck on your journey to better!

250 straight is a lot better than 260 crooked. Test a R shaft, but I bet you won’t see enough difference to switch.