The way you describe it, it seems almost like there is only one way to hook up this system. I would advise you to think fluidly about this. I often think 'what if I would patch up X to Y instead of X into Z'. Oftentimes that leads to hours of fun and exploration.
I try to maintain the limitation of the 104hp 4u case
Why? I think you'll find that a system with a bit more room will open up your module options and may even be less expensive in that way. You'll also have a better user experience if you can really get in there. If case price is a problem, you may consider building your own case. It's not that hard. Or maybe you have a friend who can work with wood or laser cutters or 3D printers who can help you make one.
A non-Eurorack sequencer can also help you here like Beatstep Pro, Korg SQ64 or Oxi. I'm assuming two Steppy and Melodi would be about 450 € used, you can get a Beatstep Pro for 135 € used and then you have one extra melodic channel more and some 60 HP (1U+3U) room in your case.
but find that I have roughly 15 mins of playability. After that it feels like I am repeating myself
It seems like a bit of a waste to me to have so many modules double and it does not surprise me that repetition is around the corner. Two Steppy, two C4rbon, two SYO.5, two Pip filter, two Peaks. No two modules, even if they serve the same purpose, are alike and can give you different ideas or ways of working and interacting.
In general
* LFOs keep things fresh, one is extremely limited for the amount of modules you have. Eowave Zone B.F. is pretty thin. Xaoc Batumi is a popular choice.
* Some DJ crossfader 1U could be cool instead of the switch.
Have you considered
* Remove one Steppy -> Doepfer clock divider / VPME - Euclidean Circles / Non-Eurorack sequencer
* Remove one (or two) SYO.5 -> VPME Quad Drum Voice / Endorphin.es Two of Cups / Noise Engineering Basimilus / Try out all Tymp firmwares and maybe find something different you like there
* two C4rbon, two SYO.5, two Pip filter, two Peaks -> Surely there's other interesting filters and VCOs out there.