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?

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

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.

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.

So it sounds to me like you may have to deal with 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.

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.

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?

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.