However, they do have some issues every now and then which really makes me question the reliability of this provider.
I would say, you can go with Mail2Tor as they have been one of the highly used mail services from the TOR community and they have been doing great for all these years. If the mail is needed for personal security reasons, i would blindly go with Protonmail (signed from clearnet - which doesn't require a mobile number for verification) Protonmail offers greater lengths of security while comparing the other mail providers. But other than that, Mail2Tor doesn't have any logs of the relays in any of their servers hence even during adverse conditions of being seized only the mail software would be compromised while the user data is being stored in a hidden TOR node.
Which one would you personally choose between Onionmail, Mail2Tor and SecMail?
I would go with Mail2Tor as I didn't face any problems associated with it for past half a year of usage. But note that, I would always go with Protonmail (signed from clearnet) for personal usage. Why don't you sign up for Protonmail from a clearnet public Wi-Fi? If you don't go with that, Mail2Tor would be the perfect alternative as they have been suggested all over through various privacy centric communities.