I agree 100% that, based on long history here, the external SSD option is likely what you need.
However, so we are not overlooking something else like software interferences or RAM starvation that could contribute to the beach-balling and other signs of slowness, please post a system config report. Fortunately there is a safe, secure way to do that.
We can quickly and within the confines of these forums help you determine what issues are at play if you use EtreCheck Pro, available here:
https://etrecheck.com/index
The free version will do nicely for this purpose, although the app is worthy of our financial support.
We can see hard data about drive performance, software issues, and RAM usage. Etrecheck is the development of a long-serving and trusted contributor here expressly for displaying information in these forums to help us help you. It will not reveal any personal or secure information.
See this excellent user tip on posting text reports like EtreCheck. It is better illustrated than mine.
How to use the Add Text Feature When Post… - Apple Community