I will probably get in trouble for saying this; please be kind, am genuinely trying to be helpful.
When I was doing his kind of thing every day right across the country, I used Windows’ Remote Desktop to access WinXP boxes. I used the VPN protocol available in my Netgear DG834v3 router’s which worked really well. What was an utter pain though was the use of NAT. I hate NAT like poison but most people are used to it and do not mind it at all, so mine is a minority opinion.
This minority opinion says don’t use NAT, insist on a proper internet connection from your ISP. Either use IPv6 in which case the question goes away - yay but then there are other minor difficulties instead - or get an ISP that gives you enough IPv4 addresses for your needs.
Of course this may very well not be relevant to you as the networks you are talking to may not be ones that you control so you don’t have a choice and in this case I apologise.
I’m just pointing out my own experience when I decided to remedy things and got an upgraded network connect which had plenty of aipV4 addresses, enough for my needs. Before when I was using NAT I had got used to logging on to the remote router first to tell it to redirect the NAT translation for the firewall hole to the correct machine’s IPv4 address (no IPv6 at this time). When NAT was gone, then there was no problem of having to choose which machine I was going to talk to and then connecting to it as a second step.
If using IPv6 there is the issue of how to handle addressing of the remote machines - you have to somehow identify them so you can talk to them; that’s a subject for another thread though.