Hmmmmm, with that break down I can see why you want to add 15s. With two 15s you can hit your 1rm squat with 2x20s and 2x15s.
Since considering your weight break down, I would go 2x15s and another 2x20..........This is a 70kg jump, but would enable you to have a better selection than only adding 50kg.
Your deadlift will be better handled with another 2x20, and with the 15s you got a bit more selection to choose from plus the squats will be easy to load up.
You got lots of fractional plates up to 5kg, so don't get smaller ones at all IMO.
You need to jump the pile up 70kg Deepy, than you got all you need for a while, after that its only 20s or 25s that you will need.