Is 30 mbits of Internet enough or is that far too little?

I currently have 150mbits for 19.99, the contract is automatically extended to 39.99 in the second month. But there's the option to go down to 30 mbits for 1 year which then costs 29.99.

Do you think that would be worth it and 30 mbits will be enough?

We're talking about: surfing, lots of youtube, playing ps4 online and just holding your mobile phone, notebook and ps4 online at the same time.

Is that enough? And then you will surely have Wi-Fi well below 30 mbits?

It is also much less than that, only downloads will last forever. It doesn't matter whether LAN or Wi-Fi

You can even see 30 Mbit / s Netflix in UHD. You only need more bandwidth if the 8 gigabyte download should only take 15 minutes instead of 45 minutes.

So in normal surfing or on YouTube you will hardly notice any difference, the loading times of websites are more latency than transmission times.

The waiting times for downloads are of course longer than now.

And no, if there are not many other WLANS available in your apartment {> 50), then WLAN is not affected either. And even if the Wi-Fi is slow, it is always slow, even if you have a 1000MBit connection. The only thing that helps is to use a good access point / WLAN router that also supports 5GHz.

It doesn't matter whether LAN or Wi-Fi

When it comes to speeds, it doesn't matter.

You should try that. I have a 50 Mbit line, and that's enough for the things you want to do.