From what I understand, opening a port isn’t a risk in and of itself — it’s only a risk if the software using the port is insecure! So long as you use reliable software and take care to configure things properly (following through with instructions from a site like ArchWiki or the official documentation helps), you’re good.
CloudFlare is more for DDOS protection, which you almost certainly don’t need . You could always set up DDOS protection later on, if the need ever arises.
Your redirect idea would probably work excellently as a browser extension — there are are redirect extensions like that for Mastodon already, actually.
As for the domain… the only thing I can think of would be, like you said, a Lemmy instance.