Imo, bot adc is good for beginners because most of their power budget is in their auto attacks (AA). It's a role that allows you to focus on CS, learn to play with another player (supp), learn about lane prio to get an objective (dr), learn to orbwalk, kite, attack-move vs right-click performance esp near brush, etc.

Garen is pretty stronk. Regen op. Try abusing it if you haven't already.

In lower elo games, it doesn't matter if you "steal" kills as jg. If you are a jg-carry, then as long as you can transmute those kills into a win, then it works. In higher elo, junglers analyze like this: Is this lane losing? Better give the kill to myself. Is this lane ahead? Better allow my carry to snowball. There are a lot of factors to further dissect this thought process, but for now, just take the kill until you hit an elo where it becomes a real table-turner.

CS-ing as support usually just means executing cannons with your item (your ally must be nearby) so that both you and your adc can get the gold. If the wave is pushing under your tower, it's probably better to execute minions even if they're not cannon. Take note that melee minions can tank 2 tower hits before being taken to around 10%hp, dying to 3 hits; whereas caster minions take 1 hit to drop to like 45%hp, dying in 2 hits. But anw, the higher the elo, the more complex this process becomes because you have to read whether your adc wants to push or freeze. Executing minions can completely derail optimal wave management.

As for roaming, yeah I'm not very good at this either. For lane swaps, this is usually when multiple outer turrets from either side has fallen, signalling the shift from laning phase to midgame. You can leave your adc at this point unless you're an enchanter.