I’ve made a few recipes with different ratios of Water to Grain for mashing in partial mash recipes. Some seem to have too little water such that the grain bag is hardly submerged. Others are higher. What is the optimal ratio for partial mash recipes specifically and do the different ratios accomplishing anything?
The mash ratio is not really essential. Your ratio of 5 L to 4 lb grain is pretty typical. Mash at whatever ratio seems right to you, then add extract if necessary, with water to hit your preboil volume, then brew as normal.
BIAB is indeed a great option and is what I do on my stovetop. I can do up to 6 gallons in 4 large pots but typically I only brew 2 gallons (8 L) at a time anyway. Batch size is totally up to your personal preference as well.
There is no one optimal way to brew. We all brew differently. The end result is the same: good beer.