So it’s (part) SAC and SSSI, for woodland habitat and associated ground flora, as well as geological interest?
My experience is more with Natural England than NRW, but I’d suggest that your move away from cutting new tracks to using existing ones would be viewed more positively. That is, as long as the existing tracks are sanctioned and accepted, not “cheeky trails” that NRW think are bad for the site.
If you want to keep going with the plan, I’d be braced for taking on the required impact assessment work to support your request, which might involve some fairly involved ecological survey work if NRW’s own survey data isn’t very up to date. NRW won’t want to undertake the development of your plans themselves for free, so it’ll be up to you to give them the right standard of information to demonstrate your event won’t be damaging and give them the confidence to sign it off. I’m guessing NRW are also the landowner?
Singletrack through woodland etc doesn’t seem that destructive, if very carefully routed to avoid the most sensitive areas and times of year (which you will have worked out based on surveys) but probably also need to consider first aid access to the course by 4×4, spectator access, where the start/finish race village will be, that sort of thing. A track record of having run “tidy” events probably wouldn’t hurt!
Good luck!