but he blindly believes that raspbian is ideal because it looks like legendary Linux
The customer is always right
You might want to look at QT, it is a bit bloated for my use, which is learning to make GUI from scratch
https://ultibo.org/forum/viewtopic.php? ... g&start=10
As for GUI's, I am still looking, pretty much gave up and decided to roll my own using OpenVG.
However customer always under estimate the time to do these things.
Raspbian has the Lite version which saves a lot of bloat and unnecessarily software.
You could try Netbeans with JavaFX, it would look familiar enough to you.
Quickie GUI's I have done with Python and Pygame, not good for 24/7 I found out, memory leaks I think?
Buildroot will get you a basic Linux OS with only what you need, only takes a weekend to learn
You don't need to use the Raspbian Linux, it has far too much for single purpose IoT use.
But it could be used as a starting point, to get up and running quicker.
"Deliver early, deliver often". Customer always use what you deliver in ways you do not expect
Netbeans or even Processing are probably going to be quickest, QT has a learning curve.
Processing has the advantage of porting to Android which you might find interesting,
You will meed Pi3 's to run these Java based tools, they can be slower.
Linux is hackable these days too, so that may be an issue for IoT stuff.
That is one major reason I moved to Ultibo.
Ultibo is not yet secure as some core stuff was just released this week and only a few know how to use it yet
Long term, tools like Ultibo, in theory can be made more secure than Linux which could have many more Zero days exploits.
But that is a lot of crystal ball gazing.
IoT is still early days and the Linux/Busybox OS's that have been used till now are hackable.
Eben said recently "Pi's will be in IoT".
They are great to learn how to do IoT and offer cheap computing to solve emerging issues.
I'm dancing on Rainbows.
Raspberries are not Apples or Oranges