Hey, I've been using grapheneos since a while. But lately I bought a pixel tablet and a Garmin watch (for tracking and notifications).

Also I bought a new SSD.
How much bigger does that my attack surface, if I don't encrypt important or sensitive data on the SSD?

And what about the tablet? For me it is kind of annoying to always check if all my apps are up-to-date both Phone and Tablet...

I don't think the Garmin watch encrypts my messages when sending it to my phone.

What I want to know is if someone (state entity for example) tries to "hack" into my devices, would I make it easier by using a Garmin watch since the messages on the watch would be in plain text, or doesn't the watch store these messages for a long time (maybe in the ram)...

So, If someone (state entity for example) would take away my phone, tablet and ssd - how big would the damage be, considering that some files on the SSD can't be encrypted since I need some files to be accessed on unencrypted devices, like on PlayStations for example.

Forgive me if it doesn't make much sense.
My first language isn't English and I don't understand much of the technical background.

Well, if your threat model includes targeted attacks by state actors (in contrary to the widely used "bulk data collection"), you have very big problems. Regarding the SSD: you could use some app to encrypt and decrypt files manually which wouldn't be very convenient - but far better than having these files not encrypted at all. Cryptomator even seems to support encrypting partitions so that may be a viable option for you. Last option would be to get a second SSD for the unimportant data (which would be for the Playstation portion) and use the first one fully encrypted.
For the Garmin watch part, I'm not sure how they handle messaging internally (maybe someone else here has experience with them) but afaik most people here don't use smartwatches and trackers if they need to connect to GrapheneOS. If you are that afraid of such an incident, don't use the watch for messages (that's what I did, and I miss nothing ^^). As of right now, the Garmin is the weakest link in your chain.
As for the "check if my apps are up to date", it's actually a non-issue for me. Aurora, F-Droid/Droid-ify/Apps/Obtanium all check for updates in the background. Almost all of them allow you to auto-update apps in the background, you just have to enable it in settings.
Hope that helps a little.