Hi! Thank you for your detailed feedback. It was very helpful. Most people have similar feedback about how the chain cooldown is too long. While it was my intention to make the player be more strategic about their chain usage, unfortunately it also reduces the feeling of responsiveness about it. I do think it's a better idea to reduce the cooldown of it by a little.
The part about losing most of your HP when setting up the chain and then running out of energy to kill enemies does feel bad and after a bit more playtesting, I've realised that. I'll be tweaking gameplay elements to strike a better balance in these mechanics. For example gaining energy the moment you create a successful link, to ensure you'll always have a supply of energy to kill chained enemies.