She's paying off the balance FOR Mike. She's worried for him and that's why she's concerned about the payment.
You've answered your own question. She's concerned about paying off the debt because they likely wouldn't be able to pay the debt off from the bar otherwise. Hence picking up side jobs like streaming and cleaning. Mike isn't really the smartest tool in the box in this regard which is why he sought out something less safe than the bank. As for the fact that the mob doesn't give out long term loans: it's a game. As I said before, you gotta suspend disbelief a bit. I've played games that force you to make a ton of money before buying an item and they feel like a slog, especially when the money making options require days upon days of going to work without the story progressing otherwise. I'm happy to be given money in excess, I worry about money in my real life, I don't want to be overly concerned about money in a game.
You have a point from a realism standpoint, but this isn't realism, it's a game. Kinda like how you go to school but there's no real reason to keep doing so unless you want two scenes. You won't lose your school funding even if you stop going.
Mike deserves some love here and there. Throw him a bone every now and again, Riley still loves him. Moreover, I'd just like to see Riley in wedding clothes/lingerie. I just don't see how we'd see her like that otherwise unless she was with Mike.
At 200 dollars a week, it will take 10 years to pay off the debt. What gangster would allow anyone that amount of time? So unless she starts earning considerably more, like thousands of dollars a week, then there's no way they'll stay alive long enough to pay it off. I mean we're already suspending disbelief, because the whole idea of a 200 dollar a week repayment scheme for a 100k debt to a ruthless gangster, is already beggaring belief. It would never happen in the real world, and Mike would already be dead in a ditch.
The real reason behind the 200 dollar a week repayment scheme, is the Dev wants this to be a slowburn corruption of Riley, and therefore she has to be able to have relatively tame jobs at the start of the game, which naturally don't pay as much. Rather than fast-tracking her into becoming a complete slut, so she can earn considerably more as a cam star, stripper or escort, and thus would be able to pay back, far more than 200 dollars a week. If she was really desperate, and was convinced that Wyatt would murder her and Mike, if she didn't pay back much more each week. She wouldn't even think twice about doing whatever it took to earn that money. She might hate herself, but she'd still do it. But because the Dev has decided this game is going to be a slowburner, instead we have this ludicrous scenario which totally stretches credibility.
Rather than just getting a loan, they could have had Mike laundering money for the Mob, until he could repay them what he owned. That would be more realistic, because then they're getting a significant benefit from the arrangement, rather than just a tiny pittance, which wouldn't even cover the high level of interest that they would naturally expect on such a sum. Because when you add in interest, Riley would likely be paying 200 dollars for the rest of her life, if her repayments remained at that level. Having Mike be involved in some criminality under duress, would also provide Riley with a big incentive to pay back the money as soon as possible. It provides a much more logical reason, for her eventually succumbing to corruption, regardless of whether Mike is a cuck or not.