So your isp is rinsing you for the cost of a an IPv4 address. £10 a month will pay for a whole /24 in 3 years.

Chances are they also skimping on other areas including over subscription. Choose a better isp if you want a better service.

Your “just open traffic to internal host 1 on your firewall is the same no matter if it has nat or not, unless you are using a non stateful firewall? Or perhaps your configuration layer splits the two for reasons.

Thank you for the advice. By any chance, have you worked with Ruby before? I remember seeing your username back when Ruby was popular and I first started learning it in university