If i may ask, few things that come in mind,
- What are the required NAS requirements in terms of availability, uptime, hardware redundancy etc ?
- What is the budget ?
- What is the workload you would like to host on the procured NAS ? SMB/CIFS, NFS, S3, structured or unstructured data ?
- What other services you would like to host on the NAS device (Docker, Office, email, backup etc) ?
- Are the above services critical for your bussiness needs ?
Avoid DIY (Thats for non bussiness critical purposes). Dont just buy on features. Stick with the one that provides the best support, things CAN and WILL go wrong at some point ! You need to have a proper support organization structured to backup your business.
Both Synology and QNAP don't offer 2 or 4 hours response time, spare part availability etc and i can guarantee that both will not engage fully to solve your issue. On the other hand, both are the titans of home to small/medium size business NAS devices. Syno is lacking some features that QNAP has BUT with Syno i've never lost data and I must say its rather difficult to do so, whereas with QNAP i've read and had some very dissapointing experiences.
So if I were to prioritize my bussiness needs :
1st. Customer support with onsite presence and spare part availability
2nd. Design a backup solution in parallel with your NAS needs.
3rd. All other mentioned features
I hope this helps a bit.
And a small comment on " And of course, we need a solid backup system (RAID) 'cause those files are super important for our clients."
RAID should NEVER be considered a backup. Its just a level of hardware failure redundancy and thats all. Plan for a proper offsite/offline backup solution as well. Backup to tape or S3, cloud or on premices targets. Ransomware and/or human mistakes can lead to loss of primary hosted data, you should be prepared for that possibility.