I think that you need to leave a wider gap, 30cm is not wide and I think it will look like what it is, ie stopping the units so they don't get in the way of the french doors. I would go with a 60-80cm gap and make a useful feature out of it with floor to ceiling shelving. I've found a picture showing some shelving stepped back from regular depth units, so you get an idea. Another option is to use a rounded base unit if you like them and maybe put a stool or plant in the gap.

Tall units up to the ceiling can look very nice, although would function as displays mostly. Honestly standard height kitchen wall units wouldn't concern me, they might accentuate the height which is not a bad thing at all. Alternatively if you have enough storage in the base units why not skip wall units altogether? Open shelves or even nothing, just a stunning splashback

How about adding brass knobs? They have a traditional look which is lovely. What colour will you be painting the doors?

I would say it’s the same for any floor covering. If you think you might want to change things in the kitchen in the future and keep the flooring then l’d go to the wall. If you’re using budget grade lino (and l know nothing is cheap these days) then it’s fine to just flick under the plinths without going all the way under the base units thereby keeping purchase costs down. Swings and roundabout scenario really.