Sorry to hear that but I think on some level it is personal. I have never experienced any of what you're talking about, everyone I've talked to has been pretty respectful and open to collaboration. It's a large project so experiences may differ, we may not have ever interacted with the same people, or you may have caught them on a bad day, etc.
On those individual things, you can certainly make apps outside flatpak although on a technical level I think that flatpak (or a similar packaging mechanism) is going to be the best option for a great number of apps, and I would expect that to become the focus for many app developers just because it's a lot easier and saves time. I think that comment about Adwaita is pretty inflammatory and may be seen as being against the code of conduct, not sure, but it certainly isn't my view and I doubt it is the view of the majority of contributors.
"neglecting basic functionality like thumbnails in the filepicker for almost two decades, even with literally hundreds of pull requests with suggested fixes."
I mentioned this elsewhere but I'm very disappointed to see this issue get continuously brought up, I don't think there is much we can say that is productive at this point. I've never seen a pull request from this that was actually finished to completion. Is there someone in particular you're waiting for to approve this? If so, can you think of something that could help them out? Or do you think they don't want help at all? Because from my perspective, that is not the case.
Edit: Also, hundreds of pull requests to implement thumbnails? Is that an exaggeration? I'd like to see a list of all of those if possible.
Keep in mind, it's not unusual for a large patch to go through many revisions before finally making it in. Take a look at the Linux kernel for another example, you don't have to look far to see many patches that take a long time to go through review or just never make it in because of various reasons. I don't think you are being fair by painting this as a GNOME behavior, it is simply reality on large projects with a lot of complexity and moving parts. It sounds like you are also saying systemd suffers from the same issues (it probably does) but unfortunately it seems that is another area where it's a complex problem space, so that's the trade-off that you make.