At least it looks as if the Malwarebytes rootkit scan might actually work. (At least, it probably works as well as anything similar.)

I see they have a standalone rootkit scanner in beta. I wonder how thorough that is?

As far as I can see, the free versions of Avast and Avira (and, presumably, other offerings) are able to scan for rootkits. Just check your settings...

It's a difficult one for the people who write security software. Take a look at this list of what's on offer:

https://www.raymond.cc/blog/10-antirootkits-tested-to-detect-and-remove-a-hidden-rootkit/

Goodness knows how anyone checks for firmware rootkits. Thank goodness they're rare. It does look as if anything which doesn't need a restart in order to launch its scan is only capable of finding user-space rootkits, but not those kernel nasties that load on boot.

I've not tried an Avast, AVG or Avira rootkit scan in ages, and honestly can't remember whether they rebooted to scan. They've probably changed since my last experience with them.