Assuming 30 vit in bonus stats but without grouping bonus (I keep getting lazy and not checking the exact group bonus):
AF1 + AV aetherial fillers + HQ amber accessories + cobalt lantern shield = 4403 hp
Full WP armor + HQ amber accessories + DL belt + WP weapon/shield = 4592hp
Full AK armor + AK accessories + DL belt + DL ring + WP weapon/shield = 4911 hp
Full DL + Myth ring + Garuda sword + WP shield = 5447 hp
Full Ilvl90 gear = 6578 hp (Guess I can tank Titan after the expansion?)
With your numbers, you will basically never tank any encounter that you can possibly receive an upgrade from. So what you're saying is that the tanks in Duty finder are there for the sole purpose of providing you with a cushy run through. You're asking to much, especially from df groups. In all actuality, the starting 4400 hp on a paladin is enough to tank all encounters pre-Titan, with Garuda being the only hard hitter. On Garuda, proper cycling of cooldowns while the damage buff add is up will still let you get by with less than 5k hp.
With that being said, few things for new tanks:
1. 30 vit as bonus stats, no ifs, ands, or buts. That's 435 of your initial hp pool, I think the only way you could have a 50 tank with less than 4k hp is if you dumped your points into something besides vitality.
2. Go to the market board, and buy amber rings, bracelet, earring and a militia choker. Get HQ if they're not to expensive, when I hit 50 I dropped about 25k on full HQ amber and a normal militia choker. They may have inflated in price since then, however at the very least grab the NQ versions (5 vit nq vs 6 vit hq I believe).
3. If you still have a lot of cash, buy an HQ Aeolian Scimitar or the warrior equivalent. You will need to purchase one of these eventually for your relic quest, however make sure you DO NOT turn in the scimitar step until you've gotten either the GC weapon, dark light weapon, or a primal weapon to replace it.
4. Shields are few and far between, if you see a cobalt lantern shield up for a decent price or you have an armorsmith buddy that can make one grab it. The materials are cheap, and the only shield before a titan kill is darklight from WP which honestly isn't worth purposefully farming over AK armor drops. Once you have all your slots filled with AK + DL and just need philo tomes you'll be doing WP anyway, so you may pick it up as a consequence of that but don't delay AK hoplite farming just to grab your shield.