But then I thought that if I want to expand in the future I'd have to buy an build a complete new rig, so I started looking into boards with more PCI-E slots, to give me room to expand later with less cost involved (simply the GPUs and another PSU). And I found these boards:
Thanks for any advice.
You are forgetting the ASUS B250 Mining Expert motherboard which can support up to 19-GPUs (with at least 6 mining GPUs minimum. these are the p106,p104,and p102s). If as you said, you are looking to expand in the future, i believe the ASUS B250 will do you good because it is much more efficient in terms of fixed and recurring costs. Good luck to you and enjoy building your mining rig buddy!