I have multiple accounts and providers and still find webmail to be the way to go. Especially if you're using IMAP the reasons to have a local client are even less.
As for an actual helpful suggestion, The Bat! Is as customizable as you could possibly want and really fast.
My favourite for many years was The Bat! by Ritlabs. Extremely powerful and it supports multiple accounts in a very instinctual way.
I just use Outlook 365 for work now, and Gmail for personal email (really good Web client and spam filtering).
This issue is resolved in The Bat! version 9.1.10, it can be downloaded from https://www.ritlabs.com/en/products/thebat/download.php
The Bat! did require "Key Encipherment" or "Key Agreement" in "Key Usage" certificate attribute for TLS even if the certificate was only used to sign ephemeral keys to provide perfect forward secrecy. If a server only supports perfect forward secrecy TLS cipher suites, the certificate used by this server may have no "Key Encipherment" or "Key Agreement" in the "Key Usage" attribute; only "Digital Signature" is needed in this case.
Google has started using a certificate that only had "Digital Signature" in the "Key Usage" attribute.
That caused the problem.
I haven't tried this in business, but at home I use The Bat, and I currently have 73GB of messages, dating back to 2000 - I never noticed any performance problems, and that's despite keeping the mail database on regular hard drive (not SSD). I really don't need all these messages, but it's less effort to let them pile up than to clean them.
you can add pop3 accounts to gmail not sure about IMAP though.
If you could get one installed...
thebat! is probably the best stand alone mail client out there but not free
Thunderbird is great (free)
of course outlook (full) is pretty good as well.
I've yet to see a webmail interface that's as fast, easy to use and featureful as a proper e-mail client.
I personally use The Bat!, but I'd use practically anything just to avoid webmail.