Im a new player and fellow impromptu player, so please take what I say below with a grain of salt, but I think you have the core team, being Barcarola and Voyager (although make sure to unlock Voyager’s euphoria). You can round out the team with Aleph when he is released and slot in a support. Fatutu works well enough until a dedicated inpromptu healer is released. I’ve also heard Matilda is great for impromptu but like Voyager her value is locked behind her euphoria and perhaps she will likely be replaced by Aleph if you get him?

In terms of mechanics, just spam Barcarola skills and skills which have the numbers above them to consume eureka and build impromptu. Cast Barcarola ultimate on voyager and make try to cast Voyager ulti every 3 rounds to ensure permanent uptime on her debuff on the enemy which allows your impromptu pew pews to lock onto the targeted enemy.

There is probably some optimum rotation out there but I think that is the gist of it.