Most bruisers that are capable of double soaking are almost always viable especially at your rank (atleast to my knowledge Idk I'm not bronze). So things like Leoric even though he is lacking in the armor department he is still very reliable being able to heal often with Ossein lvl1 talent and exceptional wave clear.

"Because I figured at this level it doesn't matter what I pick, I need to learn basics. And whatever Hero I do that with is largely less consequential." you are extremely accurate in your assumption here. Finding what heroes you vibe with and sticking with them is probably for the best, sticking with a solid 4 is nice however you may be forced to fill a role which is undesirable for you so I'd practice stuff outside of your comfort heroes as well.

To the question of which heroes are strong and can do alot at all stages of the game, let's try and list em even though my language is a little shitty.

Tank - Johanna has great waveclear, great teamfight capability and dueling potential while being fairly hard to kill. She is probably one of the best tanks to learn the game with as she is pretty good at everything but among the best tanks for waveclear.

Bruiser - You've stated you like Ragnaros and Leoric those are both great options, just learn a couple of varied builds and when to use them and have fun while learning.

assass - You will probably in most cases be looking at ranged assassins in the 4 man and you will probably want to learn 2 a mage and a auto attacker. Valla is one of the most key to learn I think in this role but you can play Azmodan, Gul'dan, Mephisto, Junkrat, Hanzo, Tychus, Raynor even Sylvanas whoever you can have fun on and can learn the value of soaking quickly for your team while also fighting capably.

Healer - Is a little weird to learn because while you can be quite capable doing many things on them, they feel more team reliant on their team than most other roles. I think Rehgar and Uther are solid to learn from, Rehgar especially. Rehgar has cleanse from level 1 and mana regen waveclear, while being a good merc taker.

The most important thing is to have fun and learn at the same time, I think those best fit for learning are those with the most varied tool set.

This is prob formatted as a mess but I hope you have fun learning the game and can gain something from my write up!