Hi Machka! I normally unclip my right foot first. I wear mtb shoes, and those spd sandals as well.
I have fallen over (thankfully usually into hedges, sometimes into nettle) at low speeds, often when climbing hills, but usually it has been as a result of mechanical failure (chain stuck, chain broke, chain into spokes. . . you get the picture).
I have hardly any miles on these new pedals (540s) so I think I will try the multi-release cleats (SH56) which are about £10 before I switch the pedals. I considered them last year when I was getting new pedals, but they were out of stock so I got a new pair of the regular cleats (SH51) to go with the new pedals instead.
Doing a search on them, these cleats seem to have helped a lot of people on the forum who were losing faith in clipless, so maybe they will save me too.