You may find the only thing holding that sleeper wall in one piece is the weight of the floor. From the pics it looks like lime mortar, sds and 6mm bit will soon shift the mortar and let you pull a brick or 6 out
PIR (solid board insulation) does have better thermal performance than rockwool (100mm PIR gives same thermal performance as 150mm rockwool) but you have to fit it tight (horribly dusty job) and per cubic metre PIR is well over twice the price of rockwool. Rockwool is easier to get airtight and has better acoustic performance, you have the depth to use it.
PIR (solid board insulation) does have better thermal performance than rockwool (100mm PIR gives same thermal performance as 150mm rockwool) but you have to fit it tight (horribly dusty job) and per cubic metre PIR is well over twice the price of rockwool. Rockwool is easier to get airtight and has better acoustic performance, you have the depth to use it.