Everything freezes, I can't use move the mouse. If I wait, sometimes I can hit Cmd-W and get the tab to close eventually, sometimes I literally have to do a hard power off.
That sounds like it could be that you're running out of memory.
Open the Activity Monitor, switch to the Memory tab, and watch the Memory Pressure graph as well as Memory Used. If memory pressure isn't green, or memory used is close to Physical Memory, you're using too much. In that state, the whole MacOS GUI can become rather unresponsive, fitting your description.
I agree with the sibling comment, what you are describing sounds like you are running out of RAM. This happens to me with Firefox on a PC with 64GB of RAM and the cursor gets janky and I have to wait until some poor process gets killed and the PC becomes interactive again and I can restart Firefox.
