I've been using Unison for over 15 years. I have my main desktop and 3 laptops. It has a simple GUI that shows which direction the file update will happen and lets me decide any conflicts if the same file has been modified in both places.
Unison is easy and consumes 0 resources in the background. It won't drain your battery scanning your folders for changes because there's more files and folders than you can get notified for by your OS.
`unison -auto` on the command line is all you need to get a convenient list of changed files and whether you want to sync them.
I have all my systems set up to sync with a low power mini PC that runs some other services.
Hasn't ever failed and deals with lots of small files and big files and everything in between without hickups or surprises.