Ok, apparently I could lower the acceleration even more! (something about different usage of comma and dots for decimals in Dutch and English...)

Previous prints (50 micron and 250 micron stacked on each other) :

(don't worry about the top, I stopped the print halfway through and had some funky temp. settings)

Same prints, but acceleration lowered by factor 10:

(took too long to print them so I cut them both of before the end.)

That looks already a lot better and I don't feel the vertical waves anymore with my nails! (though they are still a bit visible under decent lighting)
Will spend rest of week (or two weeks depending on how much free time I have) to figure out an optimal acceleration setting and then recalibrate all the other settings!

So, most of the problems I had were user-failure, but there is still some oscillation present, so any super-solutions are still welcome (and of course very interesting!)