How to deal with a compromised desktop PC

Late last week, I was using my home PC from my office via remote desktop and was kicked off with the message that another user had logged in. I didn't think much of it because I thought it was my son, even though he wouldn't normally access my desktop for anything.

Now I've found that someone has gotten into my gmail account (which has 2 factor auth and should only be possible from my home desktop where it would already be logged in) added a filter to auto-delete mail from Amazon, and then began making digital purchases such as game cards or Amazon subscriptions where you get a one time use code with my Amazon account. They also archived the orders so that they did not show up in my order list.

Because it seems the individual had access to at least one of the desktop PCs on my home network, they may or may not have accessed:

-My keypass database with all of my various account information

-Tax and other financial documents

-Other important documents with personal information

-Email and phone/text activity which I use Google Voice for

Does anyone have advice on how to recover from/protect myself from this attack?

How can I figure out how this is happening? Windows Defender scans aren't turning up anything, but logins via RDP are logged in the event viewer with the PC name, and there is currently nothing in there that isn't local or from my office desktop or my laptop.