Good you found out, thanks for sharing.

The PCIe / M.2 slot design of current PCs are simply crap.
Most consumer mainboards are only for gamers which only mount a big GPU and a lot of M.2.

And then the nonsense with up to five M.2 slots.
A better approach would have been to get more PCIe slots.
Because 1-2 M.2 storage modules you could also easily mount onto a PCIe card..
Then the user would have the flexibility to use the PCIe slots for either cards or "carrier cards" for mounting M.2.

You need to look for server or workstation mainboards, some of them offer a much better socket layout.
Last time I saw a mainboard for threadripper with a lot of PCIe sockets .. was impressive,
but those two mainboards cost between ~ 750 and 1200 €.
I found those in this offer: https://bestware.com/de/schenker-cad-st … figuration
a) https://geizhals.de/asus-pro-ws-trx50-s … 51982.html  €755
b) https://geizhals.de/asus-pro-ws-wrx90e- … 87988.html € 1169

I am glad that I found a great server mainboard from Supermicro in 2014, which still works great.
https://www.supermicro.com/de/products/ … d/X10SRi-F
It has such a cool PCIe slot design (no M.2 though), therefore I am able to upgrade so many things and still room for a HDSPe MADI FX and different USB3 cards and 2x 10 GBit LAN card with SFP+.

Today you even get no good cases so that you could mount SSDs in the front - if you like - like I did and additionally one optical drive(s) and whatever you need.

how about m.2 to PCIe riser cards, any experiences? Their price range is 15 - 50 Eur

Look into a PCIe slot extender.  Some of them are low profile enough to easily connect to the "covered up" slot and then connect the other end to your card.  There still might be a few gotchas securing your card, but it can be done.