It might be worse to try to get a replacement physical SIM as old SIM cards can cause issues like reboots as well. You could try to take it out or disable a few days to see if it makes a differenece.
Just note depending on your provider in Germany activating SIM card mobile data abroad can come with a few obstacles
Else is 5G enabled? This could cause reboots as well or VoLTE.
Is the SD card formatted as external/portable or internal extension? Especially the latter can cause various issues (out of the bkue), however SD cards can also just break in general.
So reg spare parts: I guess battery and USB port is a good idea to order, because yes to get a new battery shipped would be very difficult.