4000K is about the health of a level 80 at ilevel 580-ish who is on the road to gearing, at least out of quest Greens and getting their early Purples. If you were seeing DPS and healers with that HP, in Normals, you were in with 80s who barely need to be there. That would be discouraging for a poorly geared 73.
You are not posting on that 73 so I can’t speak to your gear specifically, but I do know that it’s very easy to level past 70 very quickly, too quickly for your gear to keep up with you.
Looking at an alt I have left behind, I would ideally like a 73 to be at about ilevel 440-450 with HP somewhere close to 1000K+, but practically I wouldn’t expect that.
HP of 700K sounds kinda like an ilevel below 400.
My first thought is the Auction House. Can you find upgrades for your weakest pieces? If your ilevel is that low, just a few good pieces should make a noticeable difference. You should be able to find crafted pieces for all armor types, ilevel 515-528
For Plate, the pieces are called Dredger’s
For Mail. Tracker’s
For Leather, Spelunker’s
For Cloth, Pioneer’s
What’s more, there is now a legitimate and convenient way to transfer gold and items between realms, called the Warband Bank - which should be called the Warbank 
. Put the necessary gold in your Warbank, make an alt on the cheapest realm, take the gold, buy the item, and put it back in your Warbank for your main to pick up.
If you are doing this, BE SURE TO DOUBLE CHECK THE TYPE AND REQUIRED LEVEL BEFORE YOU BUY. There’s no worse feeling than buying the wrong thing when you have little gold.
You can search realms on Undermine.Exchange. For example, for Dredger Plate
https://undermine.exchange/#us-area-52/search/cat=4/rmin=2/rmax=3/era=11/opt=in-stock/dredger
https://undermine.exchange/#us-area-52/222472-524
and look to the bottom of the page for prices on different realms
Of course, there could be other pieces that would work for you, and some might even be cheaper, but it should be easy to find these crafted pieces everywhere.
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Questing is NECESSARY in The War Within. While you do get XP from dungeons, you are REQUIRED to complete the main questing campaign to unlock later stuff, including World Quests. So you don’t want to do too many dungeons anyway.
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It is definitely possible to struggle through in scaled levelling, but it is no fun. When you start off behind, it doesn’t get better until you hit 80. The devs did a better job on scaling this expansion than in the last few, but it still sucks.
I’m sorry I don’t have a really good answer for where to get better gear. I think the devs feel that they did such a great job on scaling that they don’t need to provide accessible catch-up gear.