Cabinet makers don’t use wood with a prominent grain when the cabinets are going to be painted. Maple, alder, birch and even man-made boards are much better than oak, hickory, pecan or ash. Poplar is fine, but just a little more needy in terms of paint volume and prep work.

4x4s are mostly garbage for this very reason.

https://www.familyhandyman.com/article/choosing-4x4-wood/

Consider an RV converter as a charger, or run directly from a converter with no battery or the battery switched off.

You need to watch your charging voltage. With many chargers, the load of the fan will fool it into thinking the battery is depleted and raise the voltage to normal charging voltage which may cause excessive gassing and high water use. RVs get around this by having a middle charging stage where the charger operates around 13.8V when there is a small to moderate load on the system.

PVC repair is very easy and cheap. If close to the pump, replace the whole section from the pump to give yourself room to make the connections.

Not silicone! Nothing will stick to it. Not paint or even more silicone if you need to fix it later.

I would not grout that. Caulk matched to existing grout would be my preference.

If you want a line a fixed distance from the floor, just use a thin board of appropriate thickness to slide a marking device along the trim

Don’t know the look you are trying to match, but sometimes it’s better to go with a completely different color or style rather than one that is close but not close enough.

Maybe not the right answer for you, but at least worth considering.