HDMI is not so common on the Cortex-M range. If you need rich GUI or hardware media decoding/encoding probably cheap Cortex-A will be a better choice where you can run embedded linux with all whistles and bells. Well, it's not so real-time but so far in my experience there was almost no project where it was really necessary. Another approach is to use Cortex-M core for the real-time tasks and Cortex-A for the GUI - there are lot of offers where these cores are bundled on the same SoC. And it will not add costs - few days ago there was GUI capable board $7 DongshanPI-PicoW is a small Arm Linux board with SSW101B USB WiFi chip, four 12-pin headers - CNX Software[^]. Of course you need to keep in mind that such suppliers could disappear in a year. Similar TI or ST SoC that will be in production for 10 years costs x5 or x10, then you need to put it on board with all the necessary components, etc ... as you can see everything matters. Hardware is hard business.
www.codigi.net .NET touch screen GUI components suite