I would need something fully enclosed at least to go through that myself, eg putting a big motor on this one day if it ever materialises, and I hope it does

:
It's not straightforward to go about doing the right thing nor getting reasonable insurance, and even after you have gone through the correct motions and paid the price for it - you're still likely to gain unwanted attention - and since it's in the same category as a moped, you can no longer legally ride on trails or bike paths. I think that the rules are outdated and wrong. And they contribute to road congestion and pollution because for those who
can drive or ride a moped, they may as well for less faff. And those that
can't for whatever reason? They may as well continue breaking the law where ebike motor law is concerned. Let's face it that is most peoples inclination is it not.
Sorry, that hasn't helped much has it. Went on a bit of a rant. Personally I'm happy with unrestricted 250W. As long as you're considerate and responsible, avoiding making a name for yourself you can get away with about 20mph where appropriate. 30mph downhill makes me feel a bit vulnerable lol wouldn't want that to be standard.
Anyway. My advice regarding the circumstances would be don't bother and keep the speed down instead, unless you're on a nice quiet stretch