Which Google apps can be deleted or disabled without harm to the system? There's that common Google bloat ware on my smartphone's stock Android 6.0.1. I was wondering, what google apps I could savely either delete or at least deactivate without harming the OS's stability?

Device: bq Aquaris X5 Plus

OS: Android 6.0.1

All I actually need is the play store and for this, the google account. I've no intention to ever use gmail, drive, hangeouts, play music/books/games, google+ e.t.c., nor do I want to sync any data with google. 

Any help is much appreciated.
This very much depends on the device and the Android version running on it, plus where you've bought it – as the lot of pre-installed apps might differ based on that. I'm not sure about Gmail – but the other apps mentioned (Drive, Hangouts, Play Music/Books/Games, Google+) are the first ones I always disable, and never had any side effects. At least Music/Books/Games and G+ should be even safe to remove. You can also deactivate/disable the contacs sync adapter and calendar sync adapter if you don't plan to sync your contacts and calendars with Google.

What you definitely should not touch is the Google Services Framework – or you'll break "all things Google" on that device, including the Playstore.

If you want to get rid of almost all the crap, and don't need the ability to buy apps on Play or use apps that check their licenses against Google Services, flash a custom ROM and microG, plus install either Blankstore – or, if you need to buy apps and check their licenses, the original Playstore app. Details I have described in my article Android without Google: microG.