Plowing through tons of advice for fitting recreational bicycles, what can be applied to upright riding? All of it? None of it? If you ride 1 hour/day every day, fitting should still matter. Right? There must be some science for "serious" bicycles out there, where do i look?

Have one in mind?
often they are not made in more than just ONE size, so kind of like most-all Folding bikes .

I am aware of practical limits of altering your average omafiets, i am interested in the theory.
Like this: if my back has the length A (riding upright) and my arms have the length B, how do i calculate propper vertical/horizontal position for handlebar grips relative to bb or saddle?
Is anyone working on this? Or is it all going to sports?

I think you're missing the whole point of fitting, in that there is no "proper" position of anything. It's all about optimizing various elements to meet the rider's needs and abilities with regards to a particular bike.

I feel that the more upright the riding position the less fitting matters.
Basically with an complete upright position the importance of proper reach goes out of the window. Leg extension is still important but fore aft seat position loses some of its importance.

Take most mtb riders. You can ride several mtb bikes which are wildly different sizes and still be ok (this is happening with the switch from 26er to 29er and with having different purpose bikes like xc, am or enduro). Only xc bikes usually have agressive positions and even then only for actual racing.

So check leg extension and ghat you have at least the ballpark correct reach since you can't really be scrunched with an upright bike.

I feel that the more upright the riding position the less fitting matters.
Basically with an complete upright position the importance of proper reach goes out of the window. Leg extension is still important but fore aft seat position loses some of its importance.

Take most mtb riders. You can ride several mtb bikes which are wildly different sizes and still be ok (this is happening with the switch from 26er to 29er and with having different purpose bikes like xc, am or enduro). Only xc bikes usually have agressive positions and even then only for actual racing.

So check leg extension and ghat you have at least the ballpark correct reach since you can't really be scrunched with an upright bike.

Ask a framebuilder . if you are just buying a grandma's bike in the NL, which is what an omafiets, is,

then you get one in the right size for you and ride it .. like every one else does. dont need to overthink it.

It's the Long John cargo hauling bikes , such as the Bullitt, that I was thinking of..
I think the Surly Big Dummy is made in 2 sizes ..

I have a Bullit, which is more like a XC mountainbike in fit. However, it did come with a QR stem height adjustment collar. All Bullitts have the same frame size, but are definately more "sporty" when it comes to fit compared to most cargo bikes out there. Of course, with the QR stem thingy you can make it less sporty if you so wish. My GF tried my bike yesterday (her first ever go on it), and it is in fact a bit too long for her, even if I pulled up the stem. If it were her bike, I would buy a shorter stem - something like a 40-55mm I think would do the trick.

But if you really are thinking of a Omafiets, I don't have much clue to be honest. I find them to be quite difficult to get to fit the way I like it - especially for going up hills.

Not what i was thinking of, more like a simple usefull bike with decent racks.

People with racing bikes don`t need to overthink it either but some of them do, no harm in that. I don´t have a problem with my bicycle, i just like to overthink stuff and this subject seems to be poorly researched.

Generally you sit in the saddle , pull the seat post up to a decent leg extension,

while you are riding the bike, how does the reach to the bars feel ?

if too far fit a different stem maybe higher and closer [as you have a longer stem quill it will be closer] .
so maybe higher bars and a bit longer stem will suit you.

feel too cramped?, then the bars would be better further away from you..

Self examine how it feels by riding it .. and thinking about how the fit feels.

smaller frame sizes have a shorter top tube length, generally.

In NL the hills are mostly in the Maastricht province .

Research what, for whom, and why? Are the things you're talking about researching even quantifiable? In racing applications (and let's limit this to road bikes and road racing for the sake of brevity), fit theory revolves around optimizing power and aerodynamics, both of which are measurable and quantifiable. More importantly, though, those kinds of cyclists care.

So again, who cares about what, do you imagine? These 1hr recreational rides on omafiets generate the need for what kind of fit feedback?

They are probably not on omafiets, but some people ride cargobikes for a living (i.e. many hours a day). Others, like me, would like to go touring on a cargobike (a Bullitt in my case). So to us, fit is still important. I realise it is a niche within a niche within a niche, but even so, fit is important, even if it is limited to a single (or at most two) sized bike, and even if the fit can never be as tailored as with "normal" bikes.

1 hr in a lifetime is not the same as 1hr every day. People that don´t want to hurt their knees, backs, wrists etc might care. Injuries and Comfort are also measurable and quantifiable.

But the point is that there are, obviously, a huge range of geometries and fit possibilities, almost none of which result in sure injury or discomfort, so what are you talking about? You've really got a profound misunderstanding of bikes, physiology, and fit if you think there's some 'proper' formulation for bike setup; it just doesn't work that way. Bikes have to be set up to achieve something, and if your only criterion is that it be comfortable, how on Earth do you presume there is some universal positioning that's equally suitable for all people? It's impossible. No given chair, even, is universally comfortable.