I also went down the Synology 8-bay route using SHR (Synology Hybrid Raid)
It was a great decision to use it from the off IMO. Using SHR-1 it gives single disc redundancy like RAID5 but unlike a traditional RAID array I can mix drive sizes and switch drives for larger ones and, following a few rules, I get all the available drive space. When I put larger drives in the array just expands and I carry on using it as normal while it works away in the background.
I started off with 2's, 3's. 4's and 6TB drives in my NAS and now it's all 10's. I'm now looking at a 5-bay expansion module for it as buying larger discs is more expensive and isn't giving me a lot of extra room.
If you're willing to take a risk on your array all working fine and expanding SHR could save you a bit of cash as you can reuse your drives, but it could all go mightily wrong and you'll have no backup.
Regardless of how you do it, for a media server, I think SHR has lot going for it.
EDIT -
Thinking if I should get 2 x 28TB My Book Duo drives currently on Amazon for £700 and yank out the drives
I've been doing the same with the WD Mybook Duo's and shucking the drives for ages. They have WD Reds in them and it's far cheaper than buying them separately, if you buy when there are deals on they're nearly half-price sometimes compared to buying individual internal drives. You can register them on the WD website as well for warranty purposes, but I've never had one fail yet in order to try claiming.
Note that if you're going to take the risk by expanding your array and hope it all goes fine, you can't start off with the largest drives and then add smaller drives. These are some of the rules I mentioned before. You can only add drives the same size or larger than the smallest drive in the array. So if you start off with 14's, you can't use the 10's.
You'd have to copy all the stuff from the 20TB's duo to the new 28TB duo and add the 10TB discs first to the NAS.
Use this interactive tool to estimate the storage space of your Synology NAS when paired with different size hard drives and RAID levels.
www.synology.com
I'd also ask if you were planning to use the NAS as a Plex server, the QNAP would be much more suited for that as it has significantly more processing power. The Synology would struggle.