Surge XT is an open source hybrid wave-table synthesizer with a lot of 3rd party patches available. It was originally developed as a commercial product but was then released s an open source product in 2018.
It is can be installed on Windows, MAC and Linux OSs and is available through the Surge website.
Surge XT is an excellent instrument. Once you become accustomed to its unique routing system it’s fun to program and a good set of useful presets is provided. The pros on Discord have been very helpful answering questions about it.
For free synths –
U-He free versions (all of them are really good). Odin from wavewarden is very powerful.
I’ve been running into the conundrum recently too (too many synths)! I’ve decided basically to learn Vital and Phaseplant inside and out and stick with those two. I may be wrong, but it seems that between the two you can produce just about any sound you want. It’s just too much work to learn 5-6-7 different synths. With some tweaking, you can even get these digital synths to emulate some of the old analog modules.