Unfortunately your first policy is always going to be problematic.... both my girls after passing at 17 had their first year insurance on pretty boring stuff, the best insurance-wise and being dirt cheap to run was the C1.
Adding one (or both) of your parents as named drivers on your policy will lower the premium - often by 15-18%.
I think your desire to drive/run an older car while gaining experience is a great idea - including learning to drive defensively, yes, you are still at the mercy of other drivers, but aren’t we all whenever we cross the road?
Have you spoken (rather than online quotes) to members of the club insurance panel, they may be able to help. But if not, I’d probably go for the boring option, build some no-claims, get another birthday in and then try again - I know that isn’t whepat you want to hear though!
if you do find somewhere do let others know so that they can tread the same path, happy policy-hunting.
last thought, Diamond (subsidiary of Admiral) will sell you a 10-month initial policy and then give you 1 year NCB.... so you get your premium-reducing NCB 2 months early
..... Andy