[kthxbye]

Really hard to say if updating the firestick will help. I think it will still stay at Android 7 and maybe Android 7 is the compatibility issue. Without more information on what the actual error is, it's too hard to figure out.

thank you very much for taking the time to check all of this! I will look into debug logging and try to get some data. 
PS I have not updated anything system related since I rooted it, which was in like 2020, so do you think it could be some sort of incampatibility cause by kodi/spoti plugin relying on something that it outdated and needs and update?

@kthxbye

if you were capable of rooting, are you comfortable now to do some higher level debugging - it could reveal quite a lot more and even if not leading to an resolution it could mean an answer

i am not sure if your device is affected but some fire os devices are not set to output log level DEBUG, it has to be enabled from adb - see here https://forum.kodi.tv/showthread.php?tid=371987

once you are getting debug level messages, an 'adb logcat' output would show what Android is doing at the time Kodi is acting up, something that the Kodi log will not show

using grep, you can filter to the kodi process initially -
'ps -A | grep xbmc' to get the pid of Kodi
then 'logcat | grep 1234' replace 1234 with the pid to get messages pertaining to kodi

if that is not specific enough then just logcat but it could get very large and if there is a delay until it acts up the log can get very large, outputting to a file on your desktop computer (where adb is) would be recommended

Im no expert, but I will look into this after some days as I dont have time now. reall curious about having spotify inside kodi... fire stick has no its own volume control and that is the main reason, since I need to control volume level at the source. otherwise Id just use spotify app... was also gonna check out spotifyd. if it allows volume change