Thanks for pointing out another option. I never used zram before, the way I understand it is that it extends the available memory by compressing all tables in /dev/zram0 and storing them in the RAM itself. This clearly has the advantage that it does not require any actual disk space (thus leaving the already small user space untouched). Regarding the performance, I can't say which is better (reading/writing to the slower disk or doing compression in RAM).

However, I actually noticed that my swap is rarely used. I had to run a lot of apps for it to actually start running out of memory and using the swap file (and even then it only used like 1 MB from the swap). This probably has to do with the swapiness settings (vm.swappiness is set to 30). I'd mostly suspect that there is actually enough memory available on the XperiaX, but the memory management seems to be slightly off, causing the OOM handling (= killing of apps) to be executed rather fast. Extending the memory with swap of course fixes this (as there is enough free memory available), even though the swap is rarely used. If this is the case, using zram would be the better option since it's mostly about providing additional memory to prevent OOM, even though it is rarely used.