Discus breeders want to keep nutrient levels down. I bought some plants (Cryptocoryne) from a discus breeder in Christchurch who had them in a bare tank and doing 50% water changes daily and the 3 most recent leaves were white. When planted in my tank they returned to normal.
If you are growing plants you will need to retain some nutrient for them. I am in Christchurch also but only change water enough to avoid a build up of excessive nutrient that may cause algae problems.
It does not have to be at the top and spray, it can be at the bottom hidden behind driftwood or plant or you could make one like a spray from a watering can (easier to hide). It only needs to disperse the force of return water.
Discus don't have to be at high temperature, only if you want to breed them. I have a couple in a heavily planted community tank at 25 deg C and they are OK.
My aim would be to build a frame around the bottom as well as the top to distribute the load over the floor better. Any tank over about 200mm high probably exceeds the "theoretical" design floor load so it pays to make sure the load is distributed rather than all coming down through four legs of one square inch each.
They are plates of various shapes for different joining jobs and are plate steel with two sides of a triangle cut and bent up to work like nails. You bang the whole plate with a hammer and the spikes go into the wood and act like nails. There are also plates available with holes in them and nails are banged through the holes-- works the same way.
When I was breeding them I used to have 8 pairs on the go. A bare tank with a slate hanging near the surface and they always layed on that. Discus near the bottom and angels near the top. Lots of live food and a water change usually sets them off.
That would be OK. If you are serious about getting babies you should remove the parents or eggs because they will usually eat them the first few times.
It is just a matter of personal preference but I would put a 50mm strap of glass around the top to reinforce and take the flex out of the top of the tank. You can put glass lids on top of that and they will finish level with the top of the tank if you put the strap about 10 mm down from the top.
Taking into account that the glass is laminated and therefore less than 6.2mm in total I think it would be far too high. I have 10mm unlaminated on my 500mm high tanks ( and braced as well)
The natural exchange of oxygen in and CO2 out happens anyway and is improved, the greater the surface area available. The aeration ruffles the surface and increases the surface area making the exchange more eficient