Holding the highlighted red thumb key(s) will activate layers. So left thumb is nav, right is symnum, both is fun.
The shift on R is only used for shortcuts, and acts as r when tapped and shift when held. For typing I use the two sticky shift combos on ring+middle homerow.
I'm always impressed by people using these incredibly small keyboards. I've never been able to get accustomed to using anything lower than 68% for a daily driver.
Thought the same, then tried a Corne-ish Zen and ... I've been on a strange path from normal keyboard to split kbd to Ergodox to Moonlander (so more and more) down to Corne-ish Zen (3x6) and my hands/wrist thank me daily. Even today I edited my keymap https://github.com/Utopiah/zmk-config-zen-2/blob/main/config/corneish_zen.keymap thinking "Wow... so much keys left still, I could do with less".