Voxelab Polaris resin printer - 6 prints in and it's all going wrong

Greetings all - my first proper foray into 3D printing - I have a Prusa i3 clone but I've never got it printing accurately, so I got a Voxelab Polaris resin printer to play with - it's basically the same as the Elegoo Mars.

First few prints went brilliantly - used Elegoo transparent resin, 6 bottom layers with 60s cure, and 0.1mm layer height with 8s cure. The first prints were Marvel Universe miniatures, and all had flat "plinth" bottoms of about 30mm round.

I tried to print a hexagonal board game piece that was too large for the bed, so I tried to print it vertically - making it tall and thin. The print failed due to lack of support making it warp on the long axis. I added supports and tried again, but the supports snapped while printing as I think the adhesion to the FEP film outcompeted the adhesion to the bed, and I had to gently remove the cured resin from the FEP. The print bed had some stuck resin too, and I polished it off with very fine emery. Until now I've been pulling prints off the bed using a freeze spray, so the bed is not scratched at all.

Since then, I haven't had a single successful print - most common thing is the base not adhering to the print bed well enough and popping off halfway through the print.

The FEP film looks in good condition and I've cleaned it and the print bed with IPA.

I wonder if my print bed is -too- smooth now - should it have fine scratches in it to improve adherence?

Any tips would be very helpful, as I'm wasting a lot of resin trying to get this right now!

Thanks,