Those are all content issues, not format/platform issues. Don't like those? Don't make your social circle out of the people who do them.
It turns out that your very advice ("don't make your social circle out of the people who [create weird and distateful fringe content]") is the exact same echo-chamber-and-radicalization-mechanism that filled Tumblr with this kind of crap in the first place.
Anyway, I'm interested in hearing more about the radicalization thing, which I haven't actually observed first-hand, but several people in the comments here have mentioned. What interaction elements do you think lead to it?
It's not entirely optional to interact with radicalised elements on Tumblr. The tagging system lets them find you, and then they can brigade you if you fall foul of whatever pet cause they have.
This happened to me when I'd posted a photo of my dog. She's my pride and joy, and it was a harmless and cute photo of her jumping to catch a ball in the air. My mistake was using the caption "Good girl!" and tag #goodgirl. (I also tagged #dog, #fetch, #springerspaniel, #spaniel.)
I was flooded with follows from people, mostly men but also some young-ish girls, in the "daddy dom" community (their main tag seems to be #ddlg, but #goodgirl was enough for them to find me). That was a pretty disturbing subculture with definitely uncomfortable elements of abuse built right into the foundation of it.
I was also reblogged and targeted by a group of people who had decided that by "gendering" my female dog, I was a bigot. They mercilessly harrassed me. They flooded my inbox with abusive message: "dirty cis scum, kill yourself", things like that. They reblogged my photo saying some pretty vile things about me and my poor dog. In their reblogs they utterly misrepresented what I'd said, reposted it endlessly, called me transphobic, a bigot, threatened to dox me.
In the end I abandoned my entire blog there and deleted it. As far as I know, there's no way to get my dog's photo removed from Tumblr or get the hateful reblogs taken down. I reported them all for abuse and harassment, but Tumblr did nothing. Tumblr's staff don't care. They would never respond on Twitter or by email or to my reports through the form on the site. Just radio silence.
This is purely anecdotal, I know. I know someone else who also accidentally drew the ire of Tumblr and ended up having to delete their blog and leave the platform to escape a sustained campaign of harassment and threats from a similar group of people. Again, Tumblr-the-company did nothing to stop it.
I've been waiting for the GDPR to destroy them for this, it's easily the biggest abuses I can think of. Your dogs case might not be that big of a deal but many people have put up some very explicit material of themselves and I doubt they are aware that anything reblogged is still on the site.
When browsing the more explicit info it looks like 90% of the content comes from now deleted accounts, so they're in a position where compliance would destroy a major chunk of their traffic.
Not a huge loss for the world though, the UI is the worst I've come across. It takes some spectacularly bad design to make a blogging platform that doesn't work well with tabs.
Do you think it actually promotes radicalization, though, or is that just the culture that developed there? E.g. Mastodon's interaction design is very similar to Twitter's, but the people are waaaay nicer, because that's just the prevailing culture. Compare to other parts of the Fediverse, such as GNU Social, which is much more Twitter-like in tone.
Either way, I think it's a good lesson in how public-by-default social media is kind of a terrible idea in retrospect. You wanted to share a non-sensitive post, but maybe just to people who are in any way socially accountable to you, not the entire world.
It's also easier for a nice community to become mean than for a mean community to become nice, so maybe give Mastodon some time and exposure and see what happens.
... how?
300 characters incentivizes driveby remarks rather than articulate conversations.
Twitter also has really bad content enforcement. Not inherent to the format, but it's worth consideration in the interest of practicality.
(archive link because the site seems to be down at the moment)