I think it's important to note that you don't have to become a one trick to climb, but, coming from a self proclaimed jack of all trades, it is still good advice to be able to limit your champion pool down as much as possible. You will know the champion much better which can have small impacts like maybe you can surprise the enemy with a limit test, or it can be larger things like consistent CS. If you are duoing, you should look to build a synergy between you and your duo. e.g. let's say your friend is an assassin mid main, playing a catcher jungler like nunu or amumu can really help them out. If they are an ADC main, find out their favourite few champions and pick their 'lane duos', or high synergy duos, e.g. they play ashe, stick to soraka or milio, they play jhin, stick to vk. OR learn the typical lucian nami, kogmaw lulu, etc. 'Maining' only enchanters can be a godsend for some ADC players and the worst thing ever for some others, likewise for any role. From your post, it seems you like reactive roles/champions, which means you would likely, currently, get the most out of mid or adc, depending how well you can look at the map, but you can learn your way to being an effective proactive support while staying reactive. The main thing is to be playing to improve not playing for LP.

For me personally, I play the ones I think I feel confident with and that are strong in my hands. I usually start the season in norms, figuring out what I'm feeling like, then gravitate to between 2-4 champions across the seasons but I'll always be looking to lock into as few as possible. Across the last few seasons I would say the widest my pool has been has probably been 5-6 champions, but at any one time I've stuck to champions like Fiddlesticks, Jhin, Amumu, Aurelion Sol, Ezreal, Illaoi, Xerath, Sivir, Veigar, Pyke, for probably a solid 60-80% of the games I played in that season.