I think I've finally found my replacement for Thunderbird.
Claws Mail is a fork of Sylpheed, both of which are somewhat well known in the Linux world. The way to install it in Windows is by installing Gpg4win. You can skip all the privacy crap and just install the mail client straight from there.
It is not easy to set up Claws. Not at all. However it's well worth the effort to go through all the configuration crap because Claws is one fantastic frickin' mail client. It is exactly what I was looking for. A super-lightweight mail client that handles IMAP beautifully, stores messages via MailDir (each message is its own file) instead of MBOX (all messages in folder are one big file), can filter anything, purposely views mails in stripped-down fashion, is fast and just all around awesome.
Claws in a nutshell is what Thunderbird should have been. I consider it the great unsung hero of mail software that almost nobody knows about.
Like I said, this client is not easy to get along with at first, but once you get it all set up the way you want, oh yeah, it's just that frickin' good. Or at least I think so.
The best way I can describe the Claws experience is that it's like using an old-school mail client from the early-early days that has modern features in it. There is absolutely zero fluff involved. If you can imagine Alpine having a GUI on top of it, that's more or less what Claws is like - and believe me, that's not a bad thing.
Fortunately it's the case that Claws has just enough features and plugins for it to make it good to use in the Windows environment. You can make it "reply on top" by modifying the reply and forward templates, it does have a plugin called "Notification" that allows you to iconify it to the system tray, fonts are easy to change around, it handles attachments with no issues, it does plain or SSL connections easily, and so on.
You do have to really know your way around the Windows environment to figure out things like where Claws stores mail, how to back it up and so on. Almost nothing in Claws is automatic, but that's fine because it doesn't "nanny" your way through anything. This is a power user's mail client, meaning if you want Claws to do something specific, you have to figure out how to make it do what you want. Once you do, it's great.
In a few articles I read about Claws, there were actually some comments that said this is the client you want to go to if you decide to kick Thunderbird to the curb. I'd have to agree with that because there really aren't any other good free lightweight mail client options besides Claws. Well, maybe Popcorn, but that doesn't do IMAP.
I am very happy with the Claws mail client, but for the time being I'm keeping Thunderbird installed just in case I run into any snags and have to switch back. But for now, so far, so good.