We're probably at the limit of my ability to advise, but I can tell you how I handle my email. I have four mailboxes on Sonic and six on other services that primarily are for non-essential email.
I use Thunderbird in IMAP mode. This means each message is on Sonic's server until I take an action, either deleting the message or moving it to a folder on my computer.
I'm able to get to messages on the server from my computer running T-bird, my phone, my tablet, and Web Mail. T-bird has some filters (rules) built into it to automatically direct messages from certain senders or from certain mailing lists (e.g., newsletters) into folders where I can read them later. Once moved, they no longer are available via the other devices, but that's not an issue for me.
For other messages, I made a decision as I read each one:
(a) leave it in the inbox, because I need to take action on it later
(b) leave it in the inbox so I can refer to it from one of the other devices or Web Mail
(c) delete it, as I no longer need it
(d) file it in an appropriate folder within T-bird
Thus, what remains on Sonic's server is minimal, and there's plenty of time before I would hit the quota and start incurring charges.
While this works for me, it may not be the right solution for you.