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Well it seems that the actual problem is on your end. Just in case, you can compare your game to these screenshots. They were taken at the places, where you seem to have a problems: slave menu and stat list, slavery guild and all the buttons. Also, I completely disagree about your statement about mods. There is no perfect game, but mods can help you to customize your experience, making gameplay exactly as you want it to be. For example "Gigastructural Engineering" mod for Stellaris adds tons of fun staff to play around with,  but it is quite unbalanced and would be a bad idea to include in a vanilla. There are  also  tons of mods  for different games that tend to overhaul them completely, so you could enjoy your old games with a new spark of interest.  Anyway, here are your screenshots, hope you will be able to identify your problems and fix them:

1) As far as I remember, the only major thing blocked by main quest is spell shop for your character and branding of captured slaves (I am not really sure about this one, as I never really cared about quests). I guess if you want to be a mage, then yes - you are fucked. But you can still fight, unlock abilities for warrior-like character, lvl up, travel, etc. I don't really know what more do you need. 

2) You get 250 at start. Leave mansion, go to the slaver's guild and buy someone in range of 50-150 (>120 only if they are actually good). Go home and click on their names. You can choose their occupation with the middle-left button and you can see slave's stats/mood on the top. You can go to main mansion screen (Q) and click on interact button for mood stabilization.

3) On the stat list mentioned before you can see HP bar, Stamina bar and EXP bar. When it fills, you can talk to them for a lvl up. After day of work you can see that the meter fills up.

4) I don't really get this one. Buy ropes from market, exit city and hunt for more slaves. More slaves = more workers = more money. 

And just because you can't play the story, because on your own self-imposed rules, doesn't mean it should not exist. Set ration lower, get the girl, give it away and put ratio back to 100:0.

How can you find this game hard? Without main quest you won't be able to get any spells for main character, but it doesn't stop you from progressing. You can easily manage your money/food situation if you use your starting slaves correctly. It is a "management" game after all. Buy workers, make some of them earn you money or forage food, pay attention to their mood. After some hard work they will level up and you will be able to safely exit your city and hunt for slaves. I am dumb, but even I've figured out how to manage slaves in my first run, without any major issues or a need for a guide.
Quests are mainly story oriented, and tbh you can just play leveling/getting-money simulator without completing any of them (although you miss out on some nice text blocks). Game itself is pretty much a sandbox, without any real goal to accomplish. Just do whatever you like the most and enjoy it. Also all of the game's tags are fine. This is a game about managing your slave-girls(/boys), upgrading your mansion and creating biological killing machines. Story is just a nice touch, which is completely skippable, if you choose sandbox game mode.