The first thing they have to understand is the Depth of a Management Game is in Satisfying the Demands of the Customers.
The reason you Train your Girls and Upgrade your Brothel is precisely in Interacting and Manipulating the Customers and Satisfying them to get the Grand Bucks.
Bifrost does this exceptionally well since they show you the clientele beforehand so that you can plan and train for.
The problem with Brothel King is even if they have "Customers" they are generic, they are not individual with their own wants and traits and you cannot really plan for them and interact with them.
The Debt is also great because it forces you to actually Manage your business Efficiently. That's what the Challenge and Gameplay in a Management Game is to Micromanage and Optimize.
More fundamentally speaking for a Training Sim you want your Actions to have a Purpose. This is why Actions can have Tradeoffs and Costs in terms of Stamina, Sanity, etc.
Venus Blood Chimera is a good example of a pure Training Sim without a brothel.
Yakuza Boss Ririka also has good mechanics with the AV System where you need to train to particular skill thresholds and trait combinations for the scenes that increase in complexity. This could work well for a game like
Porn Empire.
The Rising Costs and Pacing of the Progression is also important. You want to double your costs or even increases it by a magnitude(10x) for the next tier(upgrades,upkeep,debt,items,girl advancement), this will keep your Economy healthy and even keep the Challenge without a Debt by locking things behind this Tier Gates, this way no matter how efficient and optimized and exploitative the player becomes you can still keep up with them.
The point is to let the Player learn and play well and have fun doing it setting up insane combo multipliers and strategies and getting the
Grand Bucks.
You don't want to be an Idle Game where you wait for the 10% improvement to pay off in X amount of time, let the player play actively setting things up and waiting for opportunities and adapting to the situation.
Racettear is also a good source of inspiration how to pace things and fun mechanics.
The other aspect of Brothel Sims and Slave Trainers is that of the Sandbox where you Freeform Customize and Tailor Your Girl for Your Tastes and Fetishes.
This can be another Mode you unlock after paying your Debt and are in the Late Game.
This is also what gives longevity and replayability with things like Image Packs for characters like you see in
Brothel King.
There is still a case to be made for giving Purpose to your Actions and still maintaining the "Game" aspect here. You still want some long term goals you want to work towards and achive higher milestones.
Maybe
TF Card Battle can also as inspiration to give you an idea for customization and replayability.
More specifically you want Your Girl you made by your own Tastes and Fetishes to be Acknowledged and have Value.
In order to do that you still need to be Evaluated and the Challenge maintained. You become the Customer, but AI Customers will also acknowledge what you Value. A relationship between brotherly gentlemen with high class.
A long term goal could be to manipulate and transform the Customers themselves to be more towards your Tastes by using your Girls and Backstage Politics to your Vision of the World. Something like
Free Cities basically.
Another aspect is that of Breeding, where you do Customization on a more Genetic Level for the purpose of creating Ubermensch children.
Portals of Phereon is the best around this in that you customize units that are used for the Tactical RPG Combat.
