What is the best and sustainable way to get rid of my old clothes and gadgets?

Have you considered passing your gadgets and clothes to others who can keep using them? This is one of the most sustainable options. Unfortunately most of the items that go to charity or other thrift shops end up in landfills. You can host a garage sale and offer everything for $1 per item.

I'd recommend using apps like Depop or Vinted. You get to make a bit of money. It doesn't take a lot of effort to create a profile and start selling your stuff. You might also consider swapping you stuff with friends.

There are a few options but it depends on what's available where you live. You can sell them on secondhand markets which are available in most countries. Be careful with thrift stores, as mentioned above some of them will end up throwing the items away in landfills or burnt which becomes a huge environmental problem. Clothes swaps with friends or neighbors are good idea too you can swap for something you wanted without spending a lot buying a new one.

You could host a swap meet or sell on local facebook groups or on valyuu.com which is mostly for used electronics.