- You're biting off quite a bit of tech you've never used, it sounds like. If you want cross platform mobile + web support, Unity is probably your best bet in terms of ease-of-use vs. portability
- I'm not sure a graphic designer is going to be able to help you with the more game-specific aspects of game graphics, but I could be wrong. Animations that work well with your game engine of choice can be a big pain
- I would buy a book or a course on some kind on mobile game monetization and marketing. It was very different from what I was used to in the SaaS space. Paid user acquisition seemed like a strange concept. Running paid UA campaigns with different install price bids based on quality of player was also a strange concept. If you want the game to make money this stuff will come up.
- Try to use services as much as you can. You're a startup, after all, you want to spend your time building the things that make your product unique. Playfab is okay but lacks decent analytics features + crash reporting IIRC. Firebase is good for mobile apps stuff but the analytics aren't great and they name stuff horribly. Regardless, pick one and try to lean on it heavily - only build the stuff you absolutely have to. Lots of tools have free trials and free tiers, use them.
- If you've never worked in games before, I would look up normal game development process. A simple version is to prototype things aggressively, then throw the bad parts out, then repeat until you're ready to do what's called a Game Vertical Slice, which usually just means to build all the "essential" parts in a way that lets you use them together. You want to flex your core loop in GVS, so you'll need each relevant piece in a usable state. When you make your GVS build, play it a lot, share it with friends, maybe try to get some friends of friends or parents involved. You want honest feedback. Is it fun? Are there parts of it that aren't? Are there things in the core loop that don't make sense? Are there gaps in the core loop? The data you'll get out of GVS is going to very aggressively scope your actual launch + success.
I say look into game development processes because I think it's important to timebox your curiousity and perfectionism. If it were a side project, I would say kind of follow your curiosity and build things exactly as you want them. Because you're trying to actually launch something, I would follow a hard structure such that you know you need to move on when you're spinning your wheels.
I'll try to think of more to write back. If you have questions I'm happy to answer them. Games are great, and making them is also great.