iMac CPU overheating: clamping pressure and thermal paste squeezed imp

I upgraded my 2019 4K imac’s CPU with an i9 9900T (35w supposed to run cooler), but am facing unexpected high temperatures with 100c and system freezes on AIDA64, or idling 60-70c.

I removed the heatsink to inspect. Tthere was the typical suction keeping the cpu attached to the heatsink initially, but the CPU had only the lightest veneer of paste on the top of the IHS.

I wondered if anybody had any thoughts about this being the cause of the issue, and any things I should try?

There is a lot of downward pressure applied with a clamp at the rear of the logicboard, I did up the bolts to the extent of their travel without overdoing them. But my first thought is that the pressure is such that it has all been pushed out.

I had applied liberal amount of Arctic MX-5 paste spread across the top of the ihs with whips and peaks, (as seems to be typical by other imac users who upgrade). I plan to do a trial run by applying a pea sized or % pattern of paste on the CPU instead of spreading it (as I would typically do).  The paste had been opened for a few months, but I have some new unopened tubes so can try a new batch in case consistency is thicker.

Another issue is that I should have used thermal pads on VRAM instead of just MX4 paste as there is too much gap - got some pads on order (not sure if this would cause the CPU to report high temps on core though).

Thank you for your time, Will