Maybe your ABS is AWOL? Get it checked

IF your braking system is working as designed, then it seems you actually want to reduce braking effriciency (or at least make it more progressive). The former is perhaps doable but more than a little dodgy for a vehicle that is to be used on public roads. Wouldn't be so bad here in Taiwan because driving tests are done on a closed test track, and bear very little comparison with real driving.

Oil (vegetable or mineral, but probably vegetable) might do it, but you'd have to experiment very gradually and carefully.

Maybe try and get some old pads, but I doubt anyone will give you them.

Asymetric shims might make the pads contact the disks unevenly (or with unevenly distributed pressure), which could reduce thier efficiency. They'll wear-in eventually and recover, but then you could remove the shims, then replace, then remove, etc.

Introduce a controlled (small) volume of air into the braking system via the bleed nipples, using a hypodermic syringe.

But seriosly. Get them checked.

A vented disc is fitted to high performance cars to improve braking from high speeds/high brake temps.

There is more to upgrading to them than simply bolting them on. You would need all the fitting hardware and matching callipers. In town where you teach they would be no advantage.

But I thought you wanted to make the braking less good, vented discs would always be seen as an upgragde to improve stopping power.

Now here is a trick. When I was competing we always used the highest performance pads we could get (for obvious reasons) but sometimes this could result in the rear of the car being overbraked (especially when the brakes were hot), the result of this being locked rears and the car disapearing off the track backwards. Some classes allowed bias pedal boxes, some allowed a valve to reduce the pressure to the rear brakes (my class did) but production classes allowed neither. A trick that many used was to saw away friction material (to reduce the contact area) until the braking balance was as required. Obviously less efficient pads may have helped but using different pad material on the rear with different heat capabilities would have its own problems.

I've found Brembo pads to have a nice light smooth progressive feel to them, they remind me of Ferodo pads of 30 years hence, normally available at decent prices from ECP/CP4L.

Make sure the sliders are lubed correctly so there's nothing sticking.

Thanks for replies also suggestion of different pads . Yes it has shoes.

Sounds more like a master cylinder or servo problem than pads alone, doesn't sound like normal brake behaviour.