Maybe adding an upgrade that makes customers arrive already with some food in their stomachs (+1 level of food per upgrade). Would not only be nice to see the "idle" customers with a bit a belly, but also specially in the later game being able to skip the lower level's of feeding, that by then give basically no real money, would be able to speed up income quite I bit I imagine. These should be decently expensive, as the potential income/sec could be quite high, certainly should be meant mainly for mid to late game
Maybe along with it also an upgrade that give passive income, per food level in each customer. This would synergies well with that previous upgrade I mentioned, and actually make adding tables actually give increased income with it as well, rather than just being a buffer of customers. Would also open up for strategies like trying to keep multiple customers at a high food level, but not fully finishing them, for more passive income(But ofc the waitress would sort of interfere, which I think is fine, making such a strategy, while potentially powerfull, still take some effort to keep up). Again mainly should be intended for mid to late game, when upgrades are costing several thousands of coins.
That said really need to fix the performance hit you get when buying new tables, specially with such upgrades added, that makes it usefull to have more tables.
If I were to make the starting weight bigger, it should be random. I think some people like to feed them from skinny to big. This is a fetish game after all. The main selling point is the weight gain but your idea could work.
Adding more tables to the game doesn't help that much tbh so your idea isn't that bad on keeping them fed enough