Optimal in what way, though? It all depends on what you're trying to achieve. I've always kept my car at slightly higher than recommended tire pressure for the firmer ride, as you mentioned; it also makes me feel more connected to the road.
I understand your hesitation in moving up to the GTS; it'll probably have a higher markup too. 4S is the minimum if you want an upgrade over the M3P. But with the 4S, there are some options that you'd want that'll already be standard on the GTS or cheaper. But 4S is definitely more engaging than...
The GTS apparently has the best range/efficiency. I've been impressed with how efficient highway driving is in my GTS. The GTS's aerodynamics must impact this a lot. I almost went to Indian Wells in my Taycan too!