I kind of wish these services were open source/core / had a self-hosted version, mainly because if they ever shut down, I wouldn't really know how to migrate out of them. But also, if they were open source/core, we'd get to add plugins / do all kinds of cool stuff on top as well.

Are there any services that are like this, but on top of say, Cloudflare Workers?

It is: https://github.com/simple-login

SimpleLogin is self hostable as aura- mentioned below.

I use simple login and think it's great.

Another self-hostable option is AnonAddy https://github.com/anonaddy/anonaddy

I think this one is as good as it gets. You can use own domains for the official server (with subscription) to retain control, and you can also self host, all while giving less technically inclined or busy users an easier option.

you can do this with Fastmail

My fear with using a service like this is that the company will vanish and I'll have hundreds of accounts tied to a dead domain. Or it gets snapped up by a bad actor. A few years ago I had my eye on idbloc.co which as far as I can tell, vanished without a trace. Their domain now redirects to Apple Music.

I wish ProtonMail offered unlimited random addresses.

how about mozilla relay

Maybe, but Mozilla's product ADHD turns me off.

How these emails will survive disposable emails validation? Currently i can't use these disposable emails in any major websites.

You can use your own domains. They give you several extras to choose from if you pay subscription. Out of hundreds of websites I used it on I've run into issues maybe twice, and they are all easily solved by using another domain they offer.