For my threat model I’m not really concerned with people gaining physical access to my phone, it’s more the surveillance capitalism I’ve got a problem with. So I’d like to be able to keep showing notification contents to myself without showing them to Google.

If properly implemented google should not see your notification “only” metadata (origin, app, timestamp).

I do not use play services on my main profile in Graphene. Here is what works for me:

Email: currently use the Proton app. I’m open to switching to a separate client, but the feature to mark read directly from the notification […]

I use FairEmail and get reliable push notifications for all my e-mail accounts. I do not have proton however, as far as I know you need the subscription to access mail via IMAP.

Calendar: also Proton, also open to other clients. Reliability here is important for obvious reasons.

I think other clients will be difficult here. Proton does not support CalDav (the open standard to sync calendars between devices using numerous applications). Proton also does not support unified push. Getting notifications here will be difficult. However, I would suggest you should try to use the protonmail app in a user profile without play services and see if you get reliable notifications. Some apps have theri own push service implemented when Googles service is unavailable (it will be more battery intensive though). If that does not work, my suggestion would be to use a diffeerent calendar service supporting CalDav (like a small selfhosted Nextcloud instance) and sync that via the android app DavX5.

Messaging: Facebook Messenger

I (have to) use Whatsapp which works okayish with notifications when battery optimization is disabled. I suspect Facebook messenger could work the same, so I would, again, suggest to just try it.

Molly is the go-to? That talks to regular Signal, correct?

Yes, Molly is a compatible fork of the official Signal client. They support Unified push but that requires running a small server to receive your Signal messages. I do however also get reliable notifications without that (more battery intensive).

Emergency/weather alerts:

I guess it depends on the country you are in. But emergency alerts via CellBroadcast are supported in Graphene and do not require Goole Play.

Microsoft Authenticator:

Have you checked if that works at all? The last time I have tried to set it up on a profile without GooglePlayServices it refused to work at all. My suggestion would be to have it on a separate profile with play services (it is healthier to separate work and private life anyways).

Smart home degoogling is Coming Soon™, if there’s anything I need to keep in mind there. Can home assistant consolidate everything or does it work through vendor apps? (Schlage deadbolts, ecobee thermostat, WiZ lightbulbs, Kasa switches if it matters.)

If a device is supported in homeassistant it should be consolidated. I would suggest looking up the ecosystem and homesssistant for details.

I’m guessing call, SMS, and MMS are a dedicated channel with the cell service and are not push notifications, but please correct me if I’m wrong.

Yes, afaik those are separate. They have worked reliably for me without any intervention or Google.

It seems like scheduled notifications e.g. alarms and timers are also not push notifications. I tried a test timer and it worked.

Yes, Googles service is only required for stuff that happens remotely and whose timing you device cannot know about in advance.

What country are you in? In my experience transit apps are somewhat localized.

This might be a jumpoff point to get Linux booting on the A18 and other apple mobile chips as well, no? On devices like the Macbook Neo or some iPads, having Linux as an option makes sense in my opinion.