Need some advice on my NAS situation.

I need some advice on my NAS situation. I've got a hand-me-down QNap TS-453BU-RP that's been working fine for the majority of the time I've had it. It's for 4x2TB spindles and 2xM.2 SSDs. The M.2's are on a PCI expansion card.

The past couple months I've been having problems where one of the M.2 drives will randomly disconnect. A cold restart doesn't seem to help (power down, remove power, wait, power back up). If I pull the thing open and reseat the drives that fixes it the majority of the time. It's usually slot 2, but slot 1 has disappeared before as well. I've tried swapping the drives around and I've tried replacement drives. None of it seems to make a difference. I've also reseated the pci card several times.

So my questions are 2 fold: First, any ideas on this issue that I maybe haven't thought of? I don't think it's the drives. Could be the PCI or the riser but I'm not sure how to go about identifying that.

Second, assuming I'm SoL with the QNap, what are my replacement options? One of the things I like about the qnap is it can do tiered storage between the SSDs and the spinning disks, presenting that all as one pool. Do any of the alternatives (TrueNAS, Unraid, etc...) have a similar feature?

This thing is a 1u chassis and I don't have a rack. Space is a premium so any suggestions that have a minimal footprint would be ideal (i.e. not a full tower case sitting next to my PC). Price is also going to be a major consideration.

Thanks