I've been using Thunderbird for a couple of years, then I went for Icedove for a while (but honestly, I used mutt more during that period) and then back to Thunderbird since 2018(?) and honestly I don't get what is wrong with the UI? At least compared to Icedove it feels very modern and fresh. I've also tried Apple Mail for a couple of weeks but that felt it was lacking both in functionality and UI experience compared to Thunderbird. But I'm just a simple user that want's to use a couple of Imap-mailboxes together with GPG and S/Mime, office administration and/or computer gaming is not my department. I've been using this MUA in both Linux and Darwin environments and it is way above the Outlook/Office 365 experience that some hostile environments force upon its users.