for the 15gb limit it would be sufficient to just get a VM with enough space (in a datacenter or at home, maybe a rapsberry pi) and run an imap , an mta and something to fetch the mails from google so that they are archived and dont fill in the limited space. i think if i were you, i would begin with just that cz that is the annoying thing and it is always possible to change the setup as wished once it is under your control.

i personally would not want to use mailcow but dovecot, postfix and fetchmail directly. fetchmail gets the mails from google and places it into dovecots imap storage while postfix would be used to send mails through google to the outside world using your google credentials. then you’ld have google as the external service to begin with and your server to actually host the emails and configure the phones to send emails through it or directly through google but just get the emails from it and save sent mails there. later you could add another nongoogly service so that fetchmail gets these emails too and just extend the setup.

if you have that, you can send/receive emails when you are at home.

but before downloading (moving) the first mails from the google storage to there i would ensure that an (incremental) backup is already running well and automatically just in case of disk failures.

But it was insecure in that you can easily go find my IP address and my real address. I don’t want that, don’t really mind if someone knows it, but I don’t want to be spearphished.

i have pretty good experience with giving every contact a separate email alias under my domain to communicate with me. my email aliases usually are like <contactshortname>-<randomnumber>@mydomain.tld

that is for a newsletter from somecoolpage.com it would look like coolpage-61514@mydomain.tld

it is near to impossible to guess that random number so i get nearly no emails from other than my real contacts cz only they know a valid address. that alias is only used for this one thing, a contact, a shop even a friend (or group of friends). mails go all into the same inbox but when i receive spam or phishing on it, i 1. know who has leaked my data and 2. i can change the alias to a new number, delete the old alias and thus stop any future spam on that address. this way i have no extra spam filters but also near to no spam.

However your ip address can be found in any email you send in the received headers. is that what you want to prevent, or just the public ip when running an internet facing mailserver with mx records pointing to it ? with ip changes beeing a thing i guess you tried to run the mailserver behind your home internet connection, nonstatic ips are bad for email, you could get a ipv6 tunnel from hurricane electric (still free?) then have static ipv6 addresses, but google afair does not allow you to send them emails via ipv6 and thus i blocked them so they cannot send me emails via ipv6 too, so i think communicating to google victims might be a problem due to google lacking behind current tech. so your idea to use a third party service fits perfectly if you dont want to run your own public mailserver. do you have a vpn to your home network to use the homeserver from remote?