Unfortunately, it's hard for me to recommend SteamWorld Heist on iOS currently, despite how well the base game is implemented, my understanding is that it's the only version that did not receive The Outsider DLC. They show the DLC available for
Steam, Wii U, Nintendo 3DS, PS4, PS Vita, and Nintendo Switch.
I saw a post from the devs three years ago, back in 2016, about the The Outsider DLC for iOS, that "... our DLC would require you to restart the game (which Apple doesn't allow) unless we rewrite it completely.
" Given that was three years ago, and the SteamWorld franchise now has two more releases (SteamWorld Dig 2, SteamWorld Quest, as well as porting SteamWorld Dig to the Nintendo Switch last year) that have joined the DLC in not making it to iOS, my expectation is that they are more focused on the Nintendo Switch as a mobile platform option and have moved on from iOS, alas.
If you have contacts within Image & Form, I'd love to hear that they are planning a big iOS push to port all the SteamWorld games currently on Switch, to include the SteamWorld Heist: The Outsider DLC, over to iOS.