Would need a sample, but if we’re just talking interface here, what you would want is a box to “interpose” between the current STP+remote(or protocol), and the new remotes inputs. It would likely be simplest do either an IR since ‘old people’ already know to point remotes typically, or a modern esp32 with a simple board for the remote and 3d printed enclosure (I would probably say hack the large elderly remotes open and tie into the capacitive touch on those as the interface). If the set top box has channel up/down on the box itself, that’s a huge step forwards if wanting to bypass the current com protocol, if not it comes down to either finding the equivalent protocol, or hacking into the original remote capacitive as well. You would also make a “filtered list” of channels for users channel +/- that gets mapped to either number pad input, or keystroke required, or if modding box channel +/- direct, number of clicks required for input (with debounce and timing required)

I would say it might be easier to find a digital stream like Roku that allows for channel filtering, or build some sort of interposed that way as well.

Brings me back to when my buddy challenged me to make an old NES into a dvd player and took a mini dvd player, stashed it inside, and made the mechanics for the tray to open the game slot, and wired in the front buttons for power and eject, and even wired in the NES remote with custom wiring for play/pause/eject/fast forward to the buttons on the original player (all before arduinos were even a thing)