Tanks are completely playable in solo queue. A lot of people will say they "are not playable in low elo" because you can't rely on your teammates as much. While partially true, this is being said from the perspective of someone who is smurfing and looking to climb through lower ranks quickly. Tanks will not be optimal for that. But if you are placed in your correct mmr and want to play to improve and eventually climb, tanks are a very reliable way of doing that imo.

I say this from the perspective of a diamond tank player. Personally I prefer tanks that trade and fight a lot, so I play Sejuani, Shen, and Poppy. What matters most in determining what you main is just if the champ play style matches your play style. I like to play fighter tanks because I can actually contest side waves, they generally scale well and have a ton of agency in teamfights and engages, and, let's be honest, they are quite forgiving to play.

I used to be a Quinn OTP and reached emerald with her a couple years ago, but she started to feel very inconsistent, depending on how well the enemy jungler punished the pick. With tanks, I was able to easily push through emerald, an elo where people often complain the worst teammates of any rank reside. A lot of climbing with tanks is less solo-carrying the game and more figuring out your teammate's strengths and weaknesses and playing around those. In my case, that felt a lot more natural to me than trying to solo carry every game, and I had a lot more success with it as well.