@easyperson
If I am you, I go for roof insulation first. It is a one shot buy. No maintenance required in the long run.

Wall, like you mentioned, you need to repaint it to have the effectiveness maintained. It costs money in long run.

From my research, rockwool or cellulose insulation for roof is your best choice.
Painting those heat rejection coating will only be good for first few years until dust start to collect on the tiles or the paint fades.

Radiant barrier (google is your friend) isn't that good in my opinion, but you may wan to consider that too.

Then you can also have option to have second roof tiles installed under your 1st tiles. Check out Monier.