The only difficulty with other services and contact syncing is 1) getting them to sync across both Android and iPhone bi-directionally and 2) syncing custom fi

The only difficulty with other services and contact syncing is 1) getting them to sync across both Android and iPhone bi-directionally and 2) syncing custom fields. 1 can largely be solved with effort (just about) but last I checked Fastmail doesnt use the same vcard formats as Google/Apple and doesnt support custom fields in contacts. That might have changed recently and I'd be very interested to look again if it has. I even went as far as a fairly engaged back and forth with Fastmail support (reference to which is in my comment history somewhere here) to work out how they could get bi-directional syncing of custom fields to both platforms working. But they had no plans to implement it. I appreciate mine is perhaps a niche use case but I've got a lot of contacts with multiple addresses and numbers in multiple countries and short of saving maybe 5 or 6 replica contact entries per individual (which is a mess over) the workable solution was to stay within a walled garden.