On the softener, since it is the one on your list with a genuinely unresolved answer.
There are two separate concerns and people conflate them:
Salt harming the bacteria. The research on this has gone back and forth and the current weight of opinion is that at normal regeneration levels it is not the problem it was once thought to be. Not the reason to worry.
Hydraulic load. This is the real one. A regeneration cycle dumps a large volume of water into the tank in a short period. That surge can stir up the settled layers and push solids toward the outlet - the same mechanism as the drainfield problem above, arriving by a different route. It also adds meaningfully to the total volume the drainfield has to absorb.
What to do about it depends on rules where you are, in some places discharging a softener to a septic system is not permitted at all, so that is worth checking before anything else. If it is allowed, a demand-initiated softener that regenerates based on use rather than on a timer cuts the number of cycles a lot, and that is the cheap improvement.
General point for a first-time owner: the local rules are not uniform, and your area's requirements for tank size, inspection and pumping intervals are the actual answer to a lot of these questions. Worth finding out what they are before adopting anybody's list, including this one.