Chromecast is DD+ only, so on a Beam (Gen 1) you won’t get surround., everything will be stereo as you have found out. But do you really need Chromecast when the apps on your smart TV (new or old) would also work?

I see. So sonos arc is the only sonos product that works with a chromecast then.

Probably, but I have no experience with the Arc. If so, the Beam Gen 2 should also do DD+.

Again: where does the Chromcast co in if you have apps on your TV that do have surround?

I use about 10 different streaming services and only 3 of these exist on my samsung smart tv. These 3 apps that are available on my tv do not update anymore and the general ux is crappy (tv is from 2014). I'd like a setup where I can play movies in the same way every time, and not use method 1 for this app, method 2 for that app and so on. My family would go crazy 🤪. 

So I think maybe if I buy an android tv I would be able to use all 10 or at least most of those streaming services on the tv with support for 5.1 dolby digital. What spec should I look for on the android tv to utilize 5.1 with the beam without the chromecast? Dolby digital and hdmi arc? Anything else?

Ah, right. I might have read your remark “I stream movies using apps such as Netflix and others” to strictly.

For the Beam HDMI-ARC is enough, though eARC would make you more future proof. Most modern TV’s are able to put out DD from apps that work with DD+, but I would get guarantees from the seller for your specific apps (or a good return policy). My 2019 Samsung QE65Q90 works well with my Beam/ if DTS from a BD-player or DVD-player is useful for you, look for a TV that can pass through DTS, which will be introduced on Sonos in the near future.

Just to be sure, if I connect this https://www.mi.com/global/mi-box-s to my tv instead of the chromecast, would dolby digital 5.1 work on the Beam then?

No guarantees., but I get surround from my Nvidia Shield Pro - though I use the apps, I do not cast to it.

Would the sonos beam gen 2 give me 5.1 with chromecast connected to my tv?

I can't decide between buying a new android tv, an android box or buy a new beam or possibly a different soundbar

Beam Gen 2 should be able to handle DD+. But since nobody has one, you will have to wait to get this confirmed. If the Arc could do 5.1 from a Chromecast, that would be a good sign. Anybody?

The speaker doesn’t determine what the TV will pass from the chromecast, that’s the responsibility of the electronics in the TV set. It does, via ARC, tell the TV what kind of signals the Beam can decode, something that is not available in a one way signal such as optical.

As @106rallye says, if the TV is capable of sending one, the Sonos Beam Gen 2 will decode a Dolby Digital Plus signal. Sometimes, the lossy version of Atmos is carried by Dolby Digital Plus.