What I would do if I were you, personally, is start using an email aliasing service like SimpleLogin and point it to whatever free mailbox you want, maybe even just your current Gmail.
That way if your current mailbox disappears for whatever reason, you can just log in to the aliasing service and switch which mailbox your aliases point to, without having to go in and update every single account you own every single time you change email addresses.
Anyways, as far as free email providers go, I have no great solutions unfortunately. Maybe someone else will share something, but like you pointed out it’s not exactly our specialty here 
People aren’t exactly thrilled with Murena around these parts, FWIW: