There is a lot of wrong here. Hard to tell where to begin or how to explain that you got it wrong.
First of all I suggest you play more and get familiar with the changes.
You do get more powerful with gear, and you can feel it. Even small upgrades. Dungeons don't scale, don't know what you are on about there.
I have no idea what you were doing in SM GY wiping tanking as a lvl 40 warrior. Even if rest of the group was ultrabad, it must have been you who were doing something wrong. Or you received no heals or something.
You may have been a 42 warrior and at good level for the dungeon, but that says nothing about rest of your group, your choice of gear and talents. Same goes for your solo attempt. Of course you should get smashed if you try to solo SM Lib/Arm. Mage is a different story, but should not be able to solo it either.
Dungeons are not solo content. This is not one of Blizzard's single player games who happen to carry the name world of warcraft.
Some tuning here and there is required, especially in the custom dungeons. Mostly it's actually the min lvl for it though. Simple fix also.
It sounds like your gear/spec is the problem. Both will eventually let you tank raids in any spec, you are playing the most OP class and it is as true here as it is on classic. You are clearly just doing something wrong.
There are a lot of people in the group finder, but you cant see them. Very few use the group browser. Que up as tank in the dungeon finder tab and toughen up. You are a warrior. Though given what you say you should probably que as DPS only until you catch up on stuff. And it's actually one of the harder classes to gear appropriately while levelling up as you need very specific combo of stats and up to date gear, running in gear you got 10 levels ago will feel almost like cloth. Naturally as you are taking the damage. This is what you want, to feel more powerful from gear upgrades. So do it. And your gonna have a hard time gearing up if your gear is already bad and don't want to invest in AH or similar. The droprates are not kind for mail/plate. And it should be this way, it's the most populated class (bar druid now perhaps), filtering out a few who cannot cope is a good thing. Maybe warrior is not for you.
I don't mean to be negative, but the game is easy. Unless you challenge it by going into dungeon before your level/gear is truly ready. This is a very good design where you have a choice.
You already outleveled the new custom dungeons that would flatten you if you struggled with SM. Difficulty is not a factor, it's your healer that have a difficult time if your gear/spec is wack, not you.
Do not make the game easier than it already is.
It needs to be harder. One reason group finder is slow/have few entries is that people faceroll the dungeons and move on, levelling at high speeds. Not doing much content that reward gear. Often having gear lag behind.
And yeah. Vanilla was harder than the joke classic is. Turtle have already stepped toward that easymode. People are soloing/duoing stratholme. Is that the kind of MMORPG you want? As a class who can accomplish this, but now in a position where you feel you can't. You know how the classes who cannot do this feel. Should we all play only the classes who can and make it into a solo/duo game just like another Blizzard turd?
No thanks. This is/was a proper game and i pray octo devs are more sane than turtle devs have been since 2023~.
This post was awkward as what i was responding to was awkward. And I don't really want to submit it, but you need to hear something along the lines and very few will deliver it.
And it seems like the sum of it all is just, you want gear upgrades. Then go get them. Even if it's just greens, by now you should have noticed that droprates and access to greens and other stuff is way beyond classic with very friendly droprates. That blue item you want from boss inside dungeon is another story. And it's not relevant, in 5 levels you will want to upgrade it again.
It's funny seeing people go for the Mara Princess ring. Making HR runs for it and all. As if the ring matter when there are 20 options and they will all be outdated in 10~ levels. Levels that are very easy to get.
Sounds like you need some proper dungeon runs and i'd love to inspect your gear.