So I just ordered a Mavic Pro yesterday and then just today started coming across a few old post that the Google Pixel and the Go 4 app had a lot of issues. A lot of people saying that you really need an IOS device for stable performance. That's ridiculous if true. Is DJI really not showing good support for android? Has anyone built any alternative apps for the Mavic? I used to have one for my 3DR back in the day.

I don't think anyone is really sure what is happening on some android devices but my opinion is that it is due to the most recent versions of android, 7.1 and later.
I originally had a Pixel XL with android v7 and did not have issues. Then there was an android update and I found that I had to make sure I held the phone in portrait when I started the DJI GO 4 app, otherwise it would just close. Then there was another android update and I found the app started to crash while I was flying. It bothered me enough to change to an ipad mini.
This is not to say that you can't get it to work but what we don't know is if there is a clash with other android apps. Some people fly without problems but they either put it in airplane mode or turn off apps running in the background or they may be on an older version of android.
There are so many android phones and so many versions of android now and DJI just can't test every different app that people may have on their phones.
You may have to just try and see how you go.

In the beginning there was a lot of crashes, but actually the DJI Go 4 app has been pretty stable lately on my Google Pixel. I have seen some issues lately that app sometimes disconnects from drone (remote still seems connected). But I'm unsure which part of my system to blame (have tried changing usb-cable, but that didn't seem to help).

But if I was you, I would begin with your Pixel XL and see how it works.

You could also wait for CrystalSky support. I hear DJI is planning some brackets to mount their own CrystalSky monitors to the remote. Btw, DJI's own CrystalSky monitors are Android based. So that DJI shouldn't be serious about Android support is nonsense. But you can't help sometimes thinking that DJIs QA processes could be a better...