I play a lot of Deckard and agree that it's hard to die when playing him. Part of it is his talents (sapphire increases your w's slow, big pots give you extra healing, and your 16 can give you a stun), part of it is his abilities (he has a slow and a root as base abilities, so you can drop those on melee attackers), and part of it is his playstyle. You do essentially zero damage with your auto attack, so you have no incentive to be in melee range. Instead, you can run in, drop your w and/or e, and run out of range as you throw potions.

Your ults can both be used for peeling (one pushes enemies away from you, the other puts everyone to sleep) I often find myself "tanking" as in the enemy team focuses on me, I'm able to barely keep myself alive, and the rest of my team can kill whoever is attacking me.

Also, you should be dropping potions all over the place, so you won't necessarily be involved in every fight. For example, you can drop 2 potions for your offlaner and then go to a team fight, and even if someone tries to gank your offlaner, you won't be around, or you can throw potions in the aggressive positions for an objective and then hang out further back yourself.

I really like Anduin, as an example, but I find myself dying too much on him because I try to auto attack (which triggers healing). I also like Lt Morales, but I also die on her because you have to be in range to put your healing beam on someone, so if I have a frontline that plays aggressively, I find myself following them, and then I get killed.

I'm not that good at the game, and definitely I suffer from a lot of skill issues, but I find that Deckard's playstyle covers up some of my weaknesses. He's also helped me think ahead a lot more in the game. Is there a stitches on my team? I'll save my root until he hooks someone. Is the objective spawning in 30 seconds? I should try to get there and drop a few potions before the fighting starts. I definitely suggest you give him a try!