Gravel

I would agree with your thoughts on the situation. If you decide to go with play sand rinse very well and when you think you have rinsed it enough repeat it.

I would not think that rounded pea gravel would cause injuries to your loaches' snouts. I kept mine on smooth aquarium gravel for about a year before I changed to sand. Sometimes when loaches have their dominance fights, they will lock their mouths together. This can cause minor abrasions on the loach's mouth. Here is a photo of some clowns I used to own engaged in a dominance battle.

It is possible to change to sand with the fish still in the tank. I've done it before. I highly recommend pool filter sand; it is easier to clean than play sand.

After only a few days there won't be much if any beneficial bacteria to lose. I have removed substrate with fish in but a much smaller tank. I used a square plastic container to scoop out the old substrate. I did half & added some new ~1/3 pool sand with a gap between old & new. Then removed the othe half after a few days break.
If you go slow adding the sand in a small pitcher or similar you can lower it & aim fairly accurately.

Pool sand is pretty inexpensive ~$10/50lbs. Unless you have rooted plants it doesn't need to be very deep

Cute pic loachmom!