Twist grip thoughts/advice

[hamster]

I'd disagree slightly in that there are plenty of 7s freehub systems around: all the way from 1989 to 1997 it populated bikes from the midrange downwards.
You can also get 9 and even 10 speed cassettes to fit, simply omit the smallest sprocket for an 8-from-9 or 9-from-10. Use the appropriate count shifter of course.

Note that SRAM 10 and 11 is not interchangeable, nor is much 9 and 10.
https://www.cyclemaintenanceacademy.com/sram-components/sram-parts-compatibility/If anyone has tried it I'd be interested to hear.

 

[Retrorockit]

8,9, and 10 speed all used different size spacers between the sprockets. Also the small cog wraps around the end of the freehub, so the cassette will be loose if you remove that one. Some guys play that game with mid drive Ebikes. Remove a middle cog or 2, and swap it to the rear to maintain spacing. Then use a longer limit screw with the proper shifter to limit gear selection. Yes 7 speed freehubs did exist. But wide range freewheels exist from BMX oriented freewheel vendors. I edited some info. on that into my previous post.
There are plenty of ways to do this right using readily available parts without resorting to that sort of thing.