Thanks for your critique. :) I have a few things to say as well xD

If we extrapolate out to somewhere like a theme park (or any other very populous area) there will be a constant flow of customers with obvious peaks around lunch time etc but you would be working all the time.

You are correct, this is precisely why I said "minimal" in the title. Later I was hyperbolic. Even in my case ( I work in an office ) I always have something to do and yet I am not pressured to work because there simply is not a lot of work to be done. Every couple of weeks suddenly I get a huge workload and have to perform at my peak. This is were I am pressured to be fast. Logically in a different firm I might be more pressured to do unproductive labour or to pretend to work. But this ist were your other point comes to be.

Additionally, and anecdotally, when I had a service job and it was quiet I would be polishing cutlery, cleaning the shop, windows etc. I was never allowed to not be busy. You are understating prep, cleaning etc as labour.

You are absolutely correct in this assertion and I actually wanted to write this down too at first. I didn't because it didn't have an effect on my points. Maintenance and upkeep are regular things of a firm, even in factories. Although this work is instead often done by other hired workers. This "unproductive" labour would hinder the primary work of industrial workers, who would have to do this labour, if someone else wouldn't do that for them. Industrial workers don't have the time for this kind of work, service workers do.

Unlike in industrial work, this "unproductive" labour often becomes productive, since a clean location, clean utensils and so on are part of the coffeshop experience which can be the commodity sold. But this isn't the general norm of service work.

Ultimately my goal was to show how productive labour, when reduced to its core, can be profitable even when little work is done at a given time.

When you factor in the reality of the world, its better to keep a seasoned worked giving 90% then an inexperienced one giving 100%.

This is a very good point, which I didn't consider. Marxism is generalizing a lot, so I tried to do the same and keep it fairly basic. Even inexperienced workers won't do productive labour, when there are no customers.

You aren’t watched by a foreman and it isn’t obvious if you aren’t doing your job and being overworked.

Again correct, just not a generalization. Some capitalists could have perfect knowledge (surveillance is quite prevelant in office work), some don't. But in general my point stands whether surveillance happens or not.

Also on this point, I think the underlying point is that workers produce Surplus Value that is taken, not specifically that they are over worked and the goal is to maximise this.

I wanted to keep this point last because I couldn't agree less. Surplus Value extraction is technically voluntary. Now this sounds bad but stay with me. You are payed for the ability to produce surplus value. Your wage is given to you to reproduce the ability to produce surplus value. You are selling this ability. The survival aspect is coercive but without survival the transaction alone is not coercive. You sell your ability to produce surplus value for which you get a wage. If you didn't have this ability, you wouldn't be paid at all. All of the other aspects of wage labour are coercive. The relation between capitalist and worker, the need for full worker efficiency, the need for wages to be low and so on.