Rando

I am staying with FF until Orion is useable on linux. They just started development

There is also a magnify feature you can try to have it zoom in, you can find this under Controller settings> bind some button > System > toggle magnifier

I did this for a diff app. And did a back paddle that I have to hold for 3 seconds

You can change how far you zoom in or out under the regular steamdeck settings > display > Advanced > magnifier scale

I admit I have not tried this at all, but maybe check if there is a key combination to go full screen for FreeTube and map that key combination to a button! That’s what I would do