Welcome to the forums! Unfortunate you have to leave the hobby for now. There's always the future to come back!
As for the hobby itself - on things like tanks and equipment - they do not really hold their value.
A 75 gallon tank goes for 50-100 unless it's literally spanking brand new. S. Repens is really pretty, in fact all your plants you might be able to get selective buyers to purchase handful of stems at a time locally, so I would say 5-10 per handful of plants. That's if there are people looking. Your LFS will give you much lower prices but would buy them usually. You might be able to get back 25% of your RO system if you're lucky and the demand is high in your area. Not sure if the substrate is sellable. Usually people start with their own substrate for growing plants.
Your discus will definitely sell. Locally they may not pitch as high a price, but if you try aquabid or even the forums here, someone might pay a bit of money depending on their size and coloration.
Trying to sell everything together is also possible but a very niche market for full setups. In my experience people who can buy a full setup end up getting new stuff