Did you already update to the most recent version of Catalina, macOS 10.15.3? The first two versions had some bugs.
I guess the issue is more on application unstable, intimidate and the longer I use the application the issue will occur.
You cannot be sure about this without some testing. The external drive may be the problem, your current Photos LIbrary may need repairing, it might be a hardware problem, or incompatible system extensions, or an incompletely installed system.
This requires systematic testing to narrow down the problem and isolate the issue. After ensuring, that you are not running out of storage on your Mac and the external volume is compatible, try the following tests:
If all tests fail, and the problem is system wide, check for hardware problems: How to use Apple Diagnostics on your Mac – Apple Support
If only the current system remains as the culprit, reinstall the system, but not the same way, as you originally installed it. Boot into recovery and reinstall the complete system, not just the update:
About macOS Recovery – Apple Support