Is there a mail app handling maildir format on one's mac in a reliable manner? I need to handle, i.e. read, write, store and search, mail in maildir or maildir++ format.

Mail must be stored on my mac, that is, reading and storing maildir on a server, via IMAP would not be good for my use case.

I am aware Thunderbird can handle "almost-ready" maildir format, but it still has a few bugs.

Also, Thunderbird is still non completely maildir-compatible.

I am aware of this question, but it does not apply to my case, as I need to be able to store messages in maildir or maildir++ proper format on my mac.

Is there a mail app handling maildir in a reliable manner, without resorting to the terminal?
I can only suggest that you run a local mailserver on your Mac using Parallels and on there you create a virtual machine being Ubuntu Linux. Strip down the install so you have Dovecot, Postfix and Getmail, and of course the GUI bits. you can then instruct Thunderbord to read mail from  Maildir store. I had this done and it works perfectly. One caveat though is that the VM MUST be able to connect to the client. otherwise you end up with a mailstore that is not accessible by Thunderbird, nor can it retrieve Internet email. YMMV depending on Linux knowledge.