I had an idea 8 (nearly 9) years ago, "I want fish"
I worked as a glazier at the time. I had no idea how to make a tank at the time.
Work cut the glass for free (they were expecting a laugh if I failed)
It was easy, the tank lasted me 6 years. I built three within six months of being a fish keeper, a bit like you, a kid in a sweet shop?
All much smaller than your vision. Then I built my master piece after about four years.
The sizes in litres were
162 which was 100x45x45 cms (LxDxH) a nice size first tank.
60 which is 60x30x30 this tank is still going.
45 which is roughly 46x30x30 this tank is still going.
I built a temporary 5ft x18"x18" tank, this is a decent size! It's still alive.
The 900 litre (750diy) is still going it is 1.6x.85x.75 internal size in metres.
It's a monster!
I built a few others of various sizes which I disposed of during the MOVE. They were still watertight.
Some other guy, some more useful tips.