Getting back into swimming or tennis after 9 years?

So when I was 11 I was doing swimming competitively on a state level and was just about to start playing tennis more competitively... but then stopped because I was getting conscious of my body bc I was pretty chubby for a swimmer and tennis player. I was also PMSing whenever my coach told me off and yelled at me during training.

I'm 20 now and I wanna become fit again. I've definitely lost a lot of "weight" bc I'm skinny but I have a lot of body fat so I'm what people call skinny fat (if that makes sense). I just wanna tone my body, get stronger and also increase my stamina (bc girl I get so tired just walking up a flight of stairs in my university lol).

So yeah long story cut short, if i get back into swimming will it be too late for me to do it competitively again? And how many times a week do you think I will need to swim or play tennis to actually make a difference with my body?

For reference I am 5'3/160cm tall, 55kg, AU size 8 tops and 10 bottoms. I also wanna make my tummy flatter and my ass fatter 😜

Edit: my doctor also wants me to gain more muscle weight