I have to sleep on my side regardless but I found that my tongue can’t fall back into my throat if I tilt my head back so I trained myself to sleep that way. Reduced my sleep apnea a lot.
I saw they have made a product meant to hold the tongue in place while you sleep. The bad news is that it looks like a pacifier.
A trick I picked up from a friend for “mouth breathing” is to put your tongue up against the roof of your mouth and create a vacuum seal. Basically touch the tip of your tongue to the back of your upper front teeth, press your tongue against the roof of your mouth and “suck” the air out.