Just keep using Remote Desktop Connection aka mstsc.exe?

It’s even recommended by Microsoft:

Although replacements have been released, as of the release of the Windows App, Remote Desktop Client is still recommended for use.

https://en.m.wikipedia.org/wiki/List_of_Remote_Desktop_Protocol_clients

With F-droid you trust F-droid to build the binary from the developers’ source code

Not when using a self-hosted F-Droid Repo - which is the case for Ironfox.

because it takes a like 3 or 4 minutes to boot

What kind of PC is this? Does it have an SSD?