Personally I would go with the last one, I don't think it looks too cluttered, and when everything is covered in moss you won't be able to see the different bits much. Also it gives you a bigger area to attach emergents to.
I think your plan for 100% sand and epiphytes + moss is a really good one, I do this in one of my tanks, and that + crypts in another, and it works really well for me. It's much simpler maintenance, because you can easily move or remove the hardscape to do a complete clean if you need to. Another big benefit is when you remove the wood you can trim or retie or add to it super easily, and no need for awkward reaching. I think it's a really nice tank too, it's simple but the animals enjoy and use it the same as the high, in fact they can hide better in it than any other tank once those plants grow in. You could maybe add some dried leaves and seedpods in the bottom to add a bit more complexity to the sand and more varied habitat for the fish (literally all my fish and inverts LOVE leaves). It's easy to remove them to clean.
If you have the time, it is worth doing a dry start on the wood with the moss, I've found that looks the most natural and even in lowtech the growth is fantastic. If you don't have the time, then wrapping with thread tightly also works very well, glueing moss just isn't the same.
I know you want to use co2, but tbh with your plant choices I wouldn't bother. I know there is that narrative that co2 make everything more perfect and beautiful and luscious, but I don't think high-tech is the best technique for every plant. I have beautiful, luscious algae-free pheonix moss and java ferns, with no co2 at all. I have high-tech versions too, and I think the petite growth of Java fern is nicer (especially trident), and the pheonix moss has much more texture and form, with excellent dense growth in my lowtech tanks; in the high-tech the pheonix moss is larger and fluffier, and java fern is bigger and spindlier, a bit denser growth, it doesn't actually look better tbh. The growth rate of lowtech is p good too, the initial start is slower but I have good rates of growth and have to trim back regularly (not too often tho).
Plus it's frankly less work, significantly. I love having my high-tech tank but I wouldn't have more than one. Whereas I have 2 lowtechs and I'm planning a third. If you really want to do it for your one tank there then isn't any harm in that, but you just might not need to and could get just as good or better effect.