Family sub questions

Hi,

I am planning to migrate away from Proton Family and I am searching for alternatives, and found mailbox.org.

My questions are;

I have 2 adults and 2 kids in my household, plus an admin account. We have like 400ish aliases, and lots of custom domains (20+). With Proton, I can use 3 custom domains with Proton Mail and rest are on SimpleLogin, which sucks a lot. How is it with mailbox? I mean, can I move all of my custom domains to mailbox? If yes, then next question will be, how aliases work. I want my family members to use my custom domains and create their own aliases AND able to write / sent messages directly from the aliases.

I don't want to create subdomains for each of my family members to use aliasing like I did with Simplelogin, because it doesn't make any sense and also takes a lot of time and management.

Next question is, how much will it cost to me?

Final question is, how does this cloud storage work? I saw Premium version has 50 GB. Really? Is it for ants? I need TBs, not just 50 GB. As for mailbox storage I need 50 GB or 100 GB at least, but Premium says 25 GB. Is that the limit? Considering Microsoft 365 Business Basic grants 50 GB mailbox and 1 TB cloud storage just for 5,60 Euro, I fail to understand 9 Euro per user tag in mailbox