I’m not really familiar with Disroot and don’t have much thought on it.
But in my opinion larger corporate email providers have more resources to maintain security and are by necessity well supported by other services.
The lack of end-to-end encryption is also a drawback.
Otherwise they seem honest about what they’re doing and how they’re doing it. If you’re willing to try out and support the project I don’t see much reason as to why not.
Proton and Tuta both have rules against multiple free accounts in their terms of service and there are occasionally benefits and drawbacks to each. I do believe Proton has publicly stated that they only bother policing that policy if it’s abused.
Overall I would say use multiple providers.
I make one exception. A private Proton account that is never shared with anyone else and a public one that used socially can make sense. Especially if one is used primarily for things like VPN, SimpleLogin and Proton Pass and the other for Proton Mail and Drive.
There are security advantages to both a single and multiple ecosystems.
Multiple ecosystems can be use to compartmentalise risks while a single ecosystem is much more practical for implementing far higher standards of security. With 2-3 ecosystems at most it might be feasible to implement both.
Either way it’s critical to have a backup and recovery plan. What happens if you rely on a service that suddenly disappears or goes offline (even for just a day or two)? What happens if they announce a closure in the near future?
Things like local backups and registering a custom domain for email can greatly simplify any possible fallout from this sort of thing.