I'm using always-on and kill switch with Proton. I use Secure Core on my Admin profile but not my user profiles. Secure Core can get slow.
I do like Proton, and I have no plans to change, but there are 2 areas that I wish were better. One is that their client software doesn't seem great and picking faster servers or letting me have them move me to a faster server automatically when I care more about speed. Second, and related, often I just want a new, fast IP from another time zone, and doing that requires wading through lists of servers manually.
Supposedly the Linux app is due for an overhaul later this year, thankfully -- it needs it.