For what it’s worth, regarding port blocks, I had relatively good experiences with that with a local ISP here. There’s no guarantee, but many ISPs block SMTP to prevent accidental zombie botnets from sending email and not technical users, so by asking might already be enough to show that you know enough about it to be unblocked.

As for the blocks, many spamlists you can get yourself unlisted. But I don’t know what permanent range blocks may exist in some systems beyond that.

It causes way more traffic for the DNS server to use a shorter TTL, so yes, it does incur more DNS traffic. In Germany some providers will disconnect you regularly if you stay connected for too long.

Some ISPs require changes ever 24 hours and will disconnect you if needed. Also, if you set DNS to cache such a short amount of time that you can react to that in 5 minutes, you will incur way more DNS traffic which can become a problem when your site is busier. Also, even if your DNS TTL is set to a super short value, a web search suggests to me in practice there will likely be downstream clients and networks that ignore it and won’t really update in such a short time frame.

Even in an ideal DNS setup, you’re probably going to have downtimes whenever your dynamic IP changes. If only because some ISPs even force-disconnect you after a while to change your address.

No german ISP that i know of does this, it’s awful. One doesn’t even offer reverse IP ptr entries whatsoever, even if you had a static IP.

You know, what’s kind of encouraging is that I posted something similar to this complaint on reddit, and 100% of the responses were corporate apologia how it would apparently be so much work and so much more expensive to provide a static instead of a dynamic IP, or how routing through VPSes is so much better anyway. I hadn’t realized the reddit to lemmy brain drain was so bad, which seems good for decentralized morally good hosting.

Personally, I find it hard to believe that just not changing somebody’s prefix all the time would possibly cause so much technical extra effort that any additional fee is justified.