If your doorways are wide enough to allow it, put a small bike basket on the one armrest of the wheelchair.
Then any small articles you need to transport you can put into the bike basket.
If you can't get through the doorways with a bike basket, then the alternative is a shopping bag either on the back of the chair or on your lap.
Light switches can certainly be lowered, but before you have that done, check whether it might be better and cheaper to have light fittings installed that can be turned on and off using a remote control that you can have either in your pocket, or lying on a convenient surface near the door of the room where the light is.
You can buy lamps that turn on and off when you clap your hands - if you can do so, and probably voice activated ones as well.
Have you had the thresholds removed from your various indoor doorways? Or small ramps put over them?
Please get someone who can get down on their hands and knees to re-route all lamp and other flexes so that none of them cross the floor, but all follow your skirting boards and are fastened to them with cable clips. This is safer, but meams you cannot move table lamps etc. so you need to work out precisely where you want them before hand.
My mother's solution to carrying small articles was to wear aprons with very large pockets where she habitually had her purse, spectacles and lower denture, plus anything else that she wanted with her from bedroom to sitting-room or elsewhere. Sorry, I didn't mean to sound facetious, but she did rarely have her lower denture in, but kept it handy if anyone came!