Rust looking spots on most of the leaves of an Indian Guava (Psidium guajava)

My Indian Guava (18 months) has been getting more and more rust looking spots on its leaves.

Front side:

Back side:

• Is this a nutrition issue? (They are given a weak fertiliser about once every 3 months)
• A type of insect? (I couldn’t find any active bugs, and they get sprayed about once a fortnight)
• Too hot? (Days are around 36C)
• Too cold? (Nights are around 5C, but it lives in a greenhouse)
• Mould? (I mould spray every month)

I found this: What are these red spots on my guava leaves? but the pics they have seem very different to mine, so I thought it would be OK to ask separately.

I rescued this plant when it was a tiny little sapling about to be thrown out by my local hardware store (Bunnings Australia), so I’ve got quite an attachment to it and would really like to figure this out.
I’ve tried so much without effect, so I’m happy to take whatever steps people recommend to get it back to health!

Thanks in advance!