Without question: Thunderbird (short: TB) may or may not be have its problems. But its very secure and last a long time.
I use this Client since 2000 (on Windows) and due to problems with my computer in the beginning, i use it the PortableApp.com Version of TB more than 10 years.
My Mail-Archive (only important mails) is beginning in the year 2000, the whole TB-folder has a size of 1.33 GB.
TL;DR; I use TB and TB Portable over 20 years and have an actual folder size of 1.33 GB. No unsolvable problems in this time. I make heavy use of the RSS-Reader, too.
Therefore i use uBlock Origin for TB.
I use this Client since 2000 (on Windows) and due to problems with my computer in the beginning, i use it the PortableApp.com Version of TB more than 10 years. My Mail-Archive (only important mails) is beginning in the year 2000, the whole TB-folder has a size of 1.33 GB.
TL;DR; I use TB and TB Portable over 20 years and have an actual folder size of 1.33 GB. No unsolvable problems in this time. I make heavy use of the RSS-Reader, too. Therefore i use uBlock Origin for TB.