Replace the internal Time Capsule hard drive or buy some new QNAP or Synology network storage

I have a 5th generation 3TB Time Capsule (A1470), which I use as my primary router and also as my one and only network drive. I have slowly started running out of space, which has prompted me to start considering whether to:

• just simply purchase a 16TB 3.5 inch internal hard drive, like this one and replace therewith my current Time Capsule's 3TB hard drive (yes, I know how to do it) or
• to buy some new QNAP or Synology network storage of similar capacity.

My top priority when deciding is the speed wherewith I can read and write from and to the network hard drive. For example, I wanna be able to play 4k and 5k video seamlessly from the network drive. I am no expert when it comes to these things but I have read somewhere that when 2 or more hard drives are connected in RAID, the speed is much higher than one would get from a single hard drive.

I would appreciate any reasonable advice. Thank you in advance!