Somewhat cross-platform (I mostly use Windows)... I use Tailscale (or just wireguard) and RDP. RDP works great on Windows and is gaining more support in Linux. VNC works as a fallback, but is slow compared to RDP.

Not free, but I was using Remotix until they got bought out by Acronis who haven't really done much but increase the price and rebrand it (really long name "Acronis Cyber Protect Connect"). It does work on Linux and MacOS but I always found the performance to be lacking and the UI is workable but not great.