A reason stoic is a lesser mechanic to unleash is also that unleash cut down on passivity while stoic enforces passivity. As a designer you usually want to encourage action.

On the flip side this might mean the decision to use stoic is a real one IMO since giving up on a threat is different than giving up on blocking.

Yeah, it's definitely got the downside of encouraging board stalls rather than breaking it. I'm like 60% confident I can make the archetype more about stalling until the "big turn", but it's still stall-heavy in a defensive format, so I'm not perfectly happy with it. The decision between stoic and unstoic being real is the only thing I'm super excited about for this mechanic.

There's also the problem of timidity, and regret. Some players will, given an option, always play it safe. To them, playing it safe will entail making a 3/3 defender, even if there's an open board. After doing that, though, that player is bound to be frustrated every turn for not attacking with their creature.

That said, this problem is also an inherent problem in Primal Clay, and there are many people who love that card. So take that advice with a grain of salt.

Honestly I think Primal Clay is more well liked because the choice isn't linked to a mechanic, which should be viewed as all upside. While Stoic can be viewed as "all upside" (in that it gives you an additional mode for the creature), I'm more hesitant to go forward with this mechanic because of the timidity and regret for the "optimal" line of play that you mentioned. If there are ways on the card to remove the +1/+1 counter, however, the decision to choose stoic or not is lessened dramatically. So the best way to use this mechanic is to use it as a vehicle to move the game forward is to get +1/+1 counters on board to use for other spells in exchange for building a defensive board that can't (at that moment) pressure the opponent.

I think Suspend is a good example to compare to because IIRC that mechanic was lukewarmly received but a lot of players really like it - I think a lot of it has to do with that timidity and regret notion that you mentioned, where players are inclined to cast a spell like Rift Bolt during a turn where they play off curve, but might regret it later when they have 3 mana and really need to kill some creature.

This is definitely less "cool" than suspend in that it doesn't have a way to add pressure to anything once it's on the board bar some other synergistic cards... I might change it to something like "Can't attack alone" or "Can't attack unless you pay .", but that might be viewed as even more downside than this.

Uhm... is the set going to feature both stoic and bide? That's a bit much when it comes to stall encouraging mechanics + it looks like these step on each other's toes design wise as well..? Ie. A keyword you put on a creature that asks you to not attack with it.

I somehow got the impression that stoic might the replacement for bide? Is this true?

Yeah, that's correct. I'm still exploring different methods for a defensively-minded mechanic. Eventually I'll get around replacing/removing the older mechanics that I'm not using.