As selam aleykum Is the prayer valid if I tie my jacket around my hips and this emphasizes parts of the ʿawrah, for example under the stomach/navel, because the T-shirt becomes very tight? I made sure that the awrah, meaning the private areas, were well covered, but around the knees etc it was tight

Ustadh AbdulAziz al-Haqqan

Subhana Allah this is a severe waswas, you should ignore these thoughts and work on waswas may Allah reward you. Also don't wear tight clothes which will solve the problem in sha'a Allah.