1: You don’t need any Robux to maintain a playerbase; you need a solid game. You can’t buy a playerbase. Spending Robux gets players to come to your game once, if they like it, they come back, thus forming a playerbase. If you want players, you’ll need Robux, but if you want a playerbase, you’ll need a good game. I released my game (it’s not a simulator but close enough in essence) on a budget of 18K Robux from selling limiteds. In a few months, I had 400K Robux in my group. I’m not saying that my game was good (because it was actually really bad), it was just really well monetized and the general feedback I get is that they liked it because it had cool graphics compared to the competing game in the market.
2: I would go with sponsorships only. That’s what I did and it worked well, but every game is different. Don’t spend all your Robux at once, do small test runs to test your sponsorship or advertisement. If you’re not making profit, go and rework some aspects of monetization or gameplay before you spend your entire budget. Once you feel your game is in a good place, split the budget over multiple days instead of spending it all in one day.
3: Mobile and Xbox certainly helps, but is not required. My game had mobile enabled for a little while before we had a botting attack that developer stats showed to be coming from mobile, but in that time it only generated about 1% of our revenue. Again, every game is different and mine wasn’t particularly well suited to mobile. Whether or not you have mobile or Xbox support is up to you and how you feel your game would perform on those platforms.