I am always afraid to upgrade a successfully working software. I have LRTtimelapse 2020-3-10, v 5.4 build 614 on my Mac Book Pro and iMac. Can't I just stay with this?

There are two reasons to upgrade:
1) Bugs will be fixed in newer versions
2) You don't get support if you use older versions.
5.5.3 is quite well tested and stable, it's also faster and brings many other improvements.
If you experience any issues, I'm always here to help. Don't worry.