Some detergents say “septic safe” on the container. My folks bought the cheapest option which did not say one way or another, so I had to call the supplier, who then said it was septic safe. It’s a shame that in the 2020s we still don’t have transparency on what we buy.
I think liquids are heavier to transport than solids because solid detergent is more concentrated (no water). Liquid detergent (which comes in all viscocities) still has its place: for people with hard water. But apart from that I think solid detergent is the best for the environment.
There are those solid tablets which are like powder pressed together. Sometimes those are in a plastic wrapper that needs to be removed before use (yikes), and sometimes they are in a disolving gelatin like the liquid pods. But I guess the sacks of powder need not be as thick as the liquid ones.