You will have about 0.5° slacker head angle and about 5 mm higher bottom bracket per 10mm of fork length, while retaining similar bar height. I went +10 on the lyrik on my meta power and am happy with the results.
Went into the red today, 2 bars on battery, 3 miles stated in high trail, 2 in boost. Rode about 3 miles in trail and boost to finally get into flashing eco mode. About 19 miles total.
I suspect riding at my us spec 20 mph will use slightly more energy than at 15.5 mph.
If you are good with at welder, you can likely get all of the shimano parts to fit.
You shouldn't be putting loved ones on a kit bike. If anything, you should ride the kit bike when learners ride your proper ebike.
With recent firmware updates, it seems more linear. Next time, record the mileage at each bar for a rough estimate assuming a similar elevation delta per mile.