yellowing lower leafs is normal, you should be more aggressive with pruning on the lower branches, especially before flowering.
She looks like she would absolutely love a transplant into a bigger pot, 11+L deep and wide pots. I would put it in a more neutral soil, compost is hard to control if you dont have good experience with it.
If you do transplant and get some new soil just get some promix HP which is like 10 for a massive compressed block of it. Shultz makes good soil too, stay away from any soil with nutrients or fertilizers, After the transplant I would start off with a half dose of any general Veg hydro nutrient. 1 week after transplanting you could go ahead and start flowering.
This is totally optional, but since you only have one you could top it, cut above a node making sure NOT to remove the node or any of the shoots coming off the node because those will be the new tops, you can grow 2 tops, 3 or 4, it depends how many shoots you leave on top (shoots are the split leaves growing out from the corner of the fan leaves)