Thanks, I will have to keep an eye out for this when it becomes viable!. Until then I am assuming that OpenKeychain generally has good longevity and there’s no reason to ditch it unless there are new vulnerabilities discovered.
There’s definitely more than two server options. Delta Chat currently lists dozens of compatible email providers and half a dozen chatmail servers on their website, and there are several more unlisted public servers. It’s common for “decentralized” platforms from Matrix to Delta Chat to have far more users than servers. But I agree there is room for improvement, and I assume decentralization will increase as more and more chatmail servers prove their stability, however chatmail is still in its early days.
By the way, Delta Chat has now removed manual key management and is discouraging the use of custom keys. In case you didn’t already see it, you might be interested to read this Github issue and this forum thread which discuss the motivation for the change and suggest directly editing the SQLite database for people who need to use their own keys.