First, I don't consider defend weak at all, if you look at the last rounds, if people had made use of defend, even if at low coins, they would have prevented most of their deaths with barely taking any damage. You don't need to kill in battle royales until the very end, you only need to survive. Yes, I made the decision to make aggressive play more rewarding, but that doesn't change the defensive nature of battle royales.
The thing about defense is, that it's meant to protect against multiple people, too, this is something attack and steal don't do, attack can defend against multiple people stealing from you, though, but steal completely leaves you vulnerable. When you select your target with a defensive option, it becomes way too unreliable to be any useful, no matter the multiplier.
Steal "counters" everything except Attack, but it's not a simple RPS decision, either, cause you take a major risk to die really easily whenever you try to steal. Considering stealing is such a massive risk, and defend is such a low risk, the return for both is fair. Even if you take someone down with stealing, you can easily die the same turn, making it not turn out well for the person that did steal, either. That is something that also influences the balance of the decisions.
I do not dislike shieldbash though, I like the concept of it. Have to think about it more.
I think heal does belong into the game (and a form of healing is a mainstay in any battle royale, resource management more specifically and preparing/amassing resources), but I'm not sure if heal is too strong or too weak right now, the way people played at the beginning, heal outperformed defend, but the way people played last round, defend would have outperformed heal. They're different, yet similar. I'll have to observe it more. It really depends on for how much people attack on average, there is a breakpoint where heal loses to defend when being attacked and ideally heal loses to defend when attacked but is better than defend when not attacked, but not too fatal, as it's not as strong as steal and thus doesn't deserve the same level of risk.
Edit: Also want to say that I do not consider this a game of RPS. It's a Battle Royale with RPS features. If anything, it's a game of resources (and trying to avoid attention via your actions, as to not become a target and die).