I hope those worms not going to effect your betta. I am going to write what i think but if Sakura will think differently then go with her advice, she is definitely more experienced.
I would clean tank 100% every about 6-7 days but not 90%. I do it for my 5 gallon tank. You need to get rid of the debris under the gravel. Just take time to acclimate your betta to new water by adding new water in his changing cup and let him sit for about 5 min to get used to the new water. Do it for a few times. Then just slowly let him in the tank.
With worms from the plant i am not sure if they that kind of worms that live on fish. I hope not. Wait for Sakura response.
But i would take everything out from the tank , wash everything with hot water and let it sit to COMPLETELY dry out for a week it should kill them
Live plant not sure though.