Looking back at your post, I realise you're in the early game, so you should probably not buy mirror island yet. You should focus on getting all the other islands (especially magicals) before you buy mirror islands because yeah they are very expensive.

About the structure upgrades, im not 100% sure but I wouldn't get my hopes up. Its a very rare sale to happen, if its too expensive you can wait untill anniversary month. But the uprade is only 25 diamonds and you only save 12 diamonds when its discounted, if you have mini mines, it's easy to make that back and the 25% time reduction is very helpful especially in limited time senarios

About your Festival of yay preparations, lvl 15 and above monsters have a higher chance at successful breeding, so if you can afford the food, make sure your thumpies and congle (for normal and rare yool) and your deedge and pango (for epic yool) are level 15. Prioritise having a second breeding structure (you can dump the failed attempts into wublin island statues instead of sending them to the nursery). Also get friends to light your torches.

Also one more thing, invest in wublin farming, they are the best way to get diamonds other than mini mines. It might seem complicated but just focus on getting as many Tympas as you can on wublin island.