hehehe ... you missed the bit where a mate should have told you that "Test drive" = "Slippery slope"
These might not be have been on your initial list ... but ...
There is chatter that soft paint on sills can be damaged (winter grit etc.) so you may want to consider front mud flaps and PPF wrap for the bottom sills (Tesla has released a corrective Winter Pack in Canada for those bits, that might become available in UK soon).
The Model-3 is not as quiet as the Model-S, so if you fancy ultra-quiet cabin then that might be worth considering. Life in an EV is different to ICE in that regard. Enjoyment of music and just general "personal karma" is enhanced by the quiet, and every bit of extraneous noise is what you actually wind up hearing, hence you might find you want to put some effort into that.
I don't know what peoples opinion of Tesla's Sentry Mode compared to a Dashcam. Sentry Mode uses a lot of juice (relatively speaking) so no good when parked at airport for long period, or if on a range challenged journey. By comparison Dashcam runs on almost-nothing.
You'll need decent (otherwise it will die quickly in harsh car climate) USB storage for SentryMode. leave a bit in the budget for APPs too (once you have the car) to log data / schedule it to be ready for you, and so on. "
Alexa: Warm the car" ... proper first world problem ...
Worth having a look at
Abstract Ocean. They have a number of interesting bits - everything from badge variations and logo puddle-lights to ... "Vote Musk" bumper stickers!
You may find that wrap for central console is essential to avoid scratches and finger prints. Screen protector too perhaps.
I've had my Model-Ss PPF wrapped. Returned the first one after nearly 4 years and 100,000 miles and it looked as good as new - my cars almost never get washed, so for me the PPF is definitely a boon, and I have zero worries about snuggling up to a countryside hedge etc. Expensive compared to Ceramic though ... and relative to Model-3 vs Model-S price PPF probably seems expensive on Model-3. You could wrap with a unique / distinctive colour though (and just buy White and save a £grand on special paint choice (I've seen the Blue, M3, it does look good ...)