Is randoming a good way to venture out and learn new heroes?

You definitely don't need to play every hero in the pool before you start randoming. A firm grasp of the basic mechanics and the items in the shop will let you play any hero, and you will learn the important skills by playing against them. There are of course quirks to just about every hero in the pool, but that's mastery-level stuff, you just need to have some idea what the hero does, especially in games casual enough to random.

That said, before you start randoming in all pick you probably want to be comfortable with:

• Farming/last-hitting
• Multi-unit control (micro)
• Ward placement
• General jungling concepts
• Hitting skillshots
• Adapting your item build

Pros of going random

• Learn heroes fast
• Get to try heroes you would never pick if you had a choice
• See a lot of different heroes in game, so you learn how to counter them too
• Players are more forgiving of errors (in all random at least)

Cons of going random

• Games tend to be one sided especially in all random
• Some combos are fun, but often they just not work
• Frustration when you can't play hero well at all and are useless
• You will learn less about game overall, as you will focus on playing your hero correctly

I would suggest going single draft + least played + random draft. All random is kinda bad for learning as you can hit both champion you are bad with, and bad teamcomp, which means that instead of having a chance to learn you will just get stomped. Or other way, your teamcomp will be so much better than opponents that you will not notice your mistakes.

That way you have some choices and can fit into a teamcomp but you still play different heroes

One thing you should not do is to go ranked as soon as you hit lv 13. If you random as a new player, you will lose more thich means even after placement matches you will be low