ADVICE PLEASE budgie found

[Finchbreeder]

The yellow on his face is brighter than on this back = Goldenface
The Seagreen body = Single factor light blue (in Goldenface the Gene that causes the Gold causes a leaching of yellow into the body blue which makes this lovely seagreen colour that you will not find in other budgie types.)
The wings could be Clearwing (if so poor show quality) or Spangle (ditto)
A full back picture may make it easier to clarify this.
Depends on what you would like to breed what the ideal mate is.
My mother is a "rares" budgie specialist. He fits within that - hence my knowledge.

 

[Somewhere_In_The_Clouds]

That sounds adorable, goodness.

Yes he is a very pretty boy, and I'm pretty sure he knows it! He has a great personality too. I'd love to know what colour mutation he isView attachment 4153966
Another photo in better lighting of him in his temporary cage, he was being all fluffy because I accidentally woke him up lol.
He's got a lovely yellow face and then that sort of melts into a pastel creamy colour on his back and wings, a little green on his low back and that blue sort of colour on his tummy. He has faded stripes too that you can sort of see. I wonder how old he is, he's certainly adult...
I plan on maybe making our own seed mix too, I want to make sure he's getting good quality seed and a good balance, the pet store brands are less than ideal from what I've looked at so far.

In simplest terms I'd say he likely fits the "Rainbow Spangle" category.
Some kind of yellow/golden face mutation (I personally cannot id those very well), Opaline and Spangle.

Any suggestions on a good cage upgrade? I want something relatively spacious for him but I want the space to be accessible to him, rather than being tall, maybe length is better?

You want a "flight cage" because you are correct, length is better than height. "Birds fly like planes, not helicopters" is a simply and useful saying that's used to explain the reasoning behind that.
If you're aiming for the most space you're looking for a cage like this.
https://www.petco.com/shop/en/petcostore/product/aande-cage-company-flight-bird-cage-in-black
If that's not suitable for your home etc. aim for something like this (for one potentially two budgies only) but pay close attention to the dimensions as there is cages this style that are too small. You're looking for the ones that have similar wide and depth measurements to the above flight cage styles.
https://www.amazon.com/You-Me-Finch-Rectangle-Flight/dp/B074P4T56Z?th=1

 

[Finchbreeder]

The 2nd option is better for 1 or 2 birds.
The shape of the 2nd option is also just plain better because of the plane/helicopter fact of how birds fly.
If indoors place the all wire cage in a corner, birds like to feel safe, as in they can see what is approaching them. The smaller the bird the more important this is. If outdoors the cage needs to be a semi solid version of the 2nd. No more than 1 or 2 of the 6 surfaces wire.
A true (show type) Rainbow budgie, should be a Goldenface Opaline Clearwing. Mum has bred them. But due to downsizing since hitting her mid 80's is not doing so now. Some people will substitute spangle for clearwing, because clearwing is recessive and spangle is not. (so it is the easy way to get a pretty bird.)

 

[Somewhere_In_The_Clouds]

The 2nd option is better for 1 or 2 birds.
The shape of the 2nd option is also just plain better because of the plane/helicopter fact of how birds fly.

I have and use both and most budgies will still use the height of the first, especially if they're young active birds that enjoy exploring. The "area" of both cages, if you get the correct size of option two, is near the same, the first is generally wider though which actually makes them better. It's much of the same. I only got the second cage originally for my elderly budgie who kept falling too far in the big cage. If I had the choice I'd only use option one.

A true (show type) Rainbow budgie, should be a Goldenface Opaline Clearwing. Mum has bred them. But due to downsizing since hitting her mid 80's is not doing so now. Some people will substitute spangle for clearwing, because clearwing is recessive and spangle is not. (so it is the easy way to get a pretty bird.)

That is why I specified Rainbow spangle, they are different to true rainbows but spangle is also a more common mutation than clearwing so rainbow spangles are easier to breed sometimes.