Notmuch (any perhaps mu) is perhaps the silver bullet you're looking for. Easily searchable, filter friendly and very much scriptable. Many TUIs including aerc and mutt can act as frontends for notmuch - though configuration may be hard. Notmuch is also supported by 'afew' - an initial mail tagger based on conditions you set (kind of like sieve, on the client side). This makes filtering of even new mails very easy. In all, I'm pretty satisfied with that side - with notmuch providing much of the flexibility and integration.
The really messy part is the server - local maildir sync part. There are half a dozen or more tools to integrate. Mbsync, OfflineIMAP and msmtp are familiar to anyone doing 2-way syncing. But if you don't want mbsync waking up every 15mins to poll the IMAP server for new mails, or if you don't want to wait 15mins for new mail, you'll have to setup a mail push notifier that that's waiting for new mail using IMAP push protocols like IDLE. That's whole another setup, just to wake up mbsync when needed. But it's still only server to localdir sync. The reverse (using file change notifiers like inotify) requires an entirely different tool with a different setup.
To make it all worse, Google has decided that they don't need to follow the standard protocols, the same way as everyone else does (for easily guessable reasons). That means that gmail needs special sync configuration or even special sync tools like lieer.
And yet another issue is that IMAP (and SMTP) are old protocols with lots of optional features bolted on. We really need a better replacement for IMAP - something like JMAP. But very few providers, servers, sync tools (any?) or clients support it.