Or even just make Gmail’s IMAP support properly spec compliant instead of requiring third party clients to hack around its nonstandard behaviors.
Could you expand on the non-standard behaviors? Long ago I noticed that using tags in Gmail causes a mess because they seem to appear as folders on an IMAP client. I’d like to know what other issues exist with respect to its IMAP implementation.
AFAIK, Gmail's IMAP uses OAUTH2 authentication through SASL (RFC7628). Legacy email clients don't implement that RFC, but it's far from a hack.