Postbox is another alternative which supports Gmail tags. It is not open source however, it costs some money, but it’s much cheaper than Mailspring if you’re going to use it for more than 4 years (right now it’s $30).
That being said, be advised: it’s thunderbird with a coat of paint and some extra features. So tabs are a thing.
have you tried Missive? its a paid app but works well
Leaving Mailspring after using it for a year in favor of Thunderbird. One of the points I found irritating with Mailspring is the nesting of messages with suppressed title lines, TB with its conventional approach is much clearer here.
Good luck to the development team all the same…