One would imagine starting from something like paneer would be more practical, no rennet needed etc.
Anyway, second the recommendation on trying cheese-making, if anybody is looking for book recommendations, ‘The art of natural cheesemaking’ (David Asher) seems like a good starting point.
I'm not sure you can really make mysost from the whey left over from acid-coagulated cheese. I used to make cheese by straining milk kefir. I made mysost/ brunost from the leftover whey a couple of times and it was intensely acidic. So much so that I used it as a thickener in soups, rather than eat it on my toast. If you want the silken, slightly tangy tofu butter style of mysost, then I don't think you can get it without whey from rennet cheese.
I wasn't aware there is non-rennet and rennet ricotta. I was using the whey method for Mysost, and even that is pretty inedible without adding a bit of cream, so I linked that recipe.
Worth noting though that (afaiu anyway) you can't make ricotta with whey from paneer - since you've already introduced acid to coagulate, there's too little if any protein left. Use it instead in bread, or in place of water in anything that you might also add yoghurt or cream to. e.g. if you make palak paneer or mattar paneer with your paneer, use the whey instead of water to reach the desired consistency.
I also agree, paneer is a great start.
Starting with ricotta gives you a higher yield of ricotta :)