Source of truth for all my contacts is my Google account then synced to my iPhone (and iCloud) via CardDAV.

I’ve been doing it this way for years and haven’t had any duplicate and/or missing data issues across the platforms.

I tried various solutions before but I think some sync'ing is broken (including how google contacts does it), and now with the fragmentation of messengers (signal, telegram, whatsapp, linkedin) I just figured as long as I know the name of someone I can reasonably expect to have a way to reach them; albeit suboptimally sometimes if I guess the preferred network wrong)

CardDAV on Fastmail. Migrated Microsoft and Google accounts to it several years back and cleaned them up in an afternoon.

You should check out HiHello. They market themselves as a digital business card, but it’s a powerful contact manager too.

I run a nextcloud instance, and sync two installations of Thunderbird and two android phones to it. All works well.