Several factors can contribute to browning of tomatoes - particular those in pots.  so first question is how big is your pot? Yours should be in a 3-4 gallon pot.  If its too small, consider repotting.  How are you watering - tomatoes in pots lose water faster than inground due to evaporation - do you have saucers under your pots, if not add them, if you do fill them with water when you water in the morning.  did you fill your pot with potting soil not garden soil?  if growing in pots use a high quality potting soil.  If your soil isn't great you could need to add nutrients, consider a liquid kelp emulsion or a dry/granular pellet fertilizer formulated for tomatoes. Airborne diseases can cause browning consider spraying with a bacterial species:  Serenade is a strain of beneficial bacteria (Bacillus subtilis) that is safe for people and pets.  It either consumes or outcompetes a huge number of pathogenic bacterial and fungal species. take a look at this:  https://extension.unh.edu/blog/what-best-way-grow-tomatoes-container;  https://extension.psu.edu/container-grown-tomatoes.

sounds like you did everything right in terms of growing plants in pots - so its probably bacterial or fungal - this has a good chart for diagnosing problems based on leaf characteristics - you are looking at areas of dead/dry leaf blades. https://www.towergarden.com/blog.read.html/en/2017/8/plant-problems.html