Selfhosted

I would cancel the new ISP on principal. Fool me once shame on you, if they fool me twice it's on me. I wouldn't give them the opportunity to fuck me again.

Having your ISP do your port forwarding seems alien to me as that's not the norm where I am. Since it seems like a standard thing where you are, you may run the risk of another ISP doing the same thing. Personally, if the price is right, I'd take the latency hit and get a VPS and route all inbound traffic through that via wireguard.

Slap CloudFlare tunnel in front of your web services and call it a quits?

So it sounds to me like you have a IPv4 only address behind CGNAT, which makes port forwarding not work anymore. It's how my connection is set up, but luckily it does fully support IPv6 and that doesn't require any forwarding so I make do.
If IPv6 isn't an option for you or you'd like to access your services from IPv4 only networks, I'd just go with Tailscale myself. I've been a happy user for years and it just works so well, should be good in your situation as well.

I have about 25 letsencrypt certificates on the same domain, so that is definitely not an issue.

I switched to a fiber to the home ISP and only found out they do CG-NAT afterwards. I opted to go VPS with Wireguard and it has been problem free and only cost me $2.50 USD/mo.

What's the ISP? Is it one of those ISPs that do CG-NAT by chance?

It seems weird that port forwarding is even considered to be a feature on the ISP side, that's usually a router thing.

Any chance you could run your own router? Because as long as your router can connect to the ISP, and get a public IP from it, there's not much the ISP can do unless they have firewalls or a NAT system.

The only situation that makes sense to not do port forwarding is those CG-NAT ISPs and carriers.

Otherwise, yeah, you can get the smallest possible VPS possible (some can be obtained for $3-$5/mo) and you can just VPN your stuff home pretty easily.

Do you have IPv6? Just let your service’s IP/port through the firewall.

(If you have no IPv6 but CGNAT, the ISP is bad…)

A couple thoughts for you. I have a wonderful local fiber ISP and when I got hooked up, I discovered they were doing CG-NAT on residential connections. I called up and asked if I could have a public IP to host services and they just immediately gave me one. Definitely not the stereotypical ISP interaction, but if you haven’t already tried asking politely, it might be worth a shot.

On the last item, yes, letsencrypt lets you get certs for the same domain from multiple hosts, but I’ll often use a self-signed cert on the host and then get the public-facing cert at the reverse proxy level. No need to coordinate copying certs over in most cases.

If your self-served stuff is just for you or family, I use tailscale for that. Nothing publicly enabled, have to be in the tailscale net to access.

Your ISP gives you 1Gbps but doesn't give you your own IP so you need port forwarding?

Have you considered keeping both plans? You said it was a different isp - dsl and fiber use different cables is it may be possible. Depending on what youre after, this may be a fun project for tying two lines together.

Just use a cloudflare tunnel. It's free and can be used on pretty much any network that sends and receives data.

Do they not offer an opt-out of CG-NAT? or a surcharge for a static IP?

With Lemmy, it will depend more on the amount of media that your users upload than anything. With Mastodon, you also will have to consider the amount of data in the cache.

With Mastodon, I have (small) instances running for about an year or so which are using less than 100GB, and I have instances from power users that went on to take ~250GB in less than a month.

You can set up Mastodon to delete posts after a certain age and to clear the remote cache periodically. This would help mitigate both things.