Temperature issues after repaste - possible heat sink contact problem

Hey all, hope I can get some advice here.

I've had this 2009 13" MacBook Pro for a while now and have been using it to learn macOS and tinker with and it's been great. But seeing as it's 16 years old by now it's been running a bit slow and hot so I decided to take it apart and give it a new application of thermal paste. The trouble is, after the repaste the thermals now seem to be worse.

Idle and load temps are a bit higher, but more concerningly the temperature reading is much more inconsistent and "spikey"; before the repaste it would gradually increase or decrease but after repaste it can suddenly climb or drop by up to 10°C within a second which I'd never seen it do before. Even while load remains constant the temperature can suddenly fluctuate up or down within that range.

My first thought was I'd not applied enough paste, but after taking everything apart again the CPU and GPU dies were completely and evenly covered and the issue remained after repasting it again. I'm thinking maybe the heat sink isn't making enough contact with the dies but if that's the case I'm stumped because I've tightened the screws about as much as I dare (my screwdriver was starting to slip and it felt like if I applied much more torque the screws were about to start stripping). The thermal paste I used was Thermal Grizzly Aeronaut which I assumed would be fine for a direct die application like this.

Any help would be greatly appreciated. I love this little machine and want to do everything I can to keep it going!