The 20/30 discount is from shopee, classified under electronic category.

They always reset after 24 hr, so everyone including myself will go claim the vouchers on first come first basis.
https://shopee.sg/m/electronics-zone
Flash deal means usually discount 0-40 dollars, however shopee xiaomi official store dun really give discount now, just a gimmick to lure unsuspected person like you to bite.

You just observe, remember the pricing eventually you will figure that marketing strategy from xiaomi shopee.
On which phone you should buy, I will only say go for 6/128 configuration minimally now.
You can also go play shopee games on the apps and earn coins, and you should go and "claim" and earn as much coins as possible so it can help you offset a few cents off from the purchase price.
https://shopee.sg/shopee-coins/
If you using some credit cards you may eligible for additional rebate but usually those rebates will snatch up by people very fast.