I think there's something that limiting your transfer speed, maybe the flash drive, or FTP or you choose USB 2.0 mode on the router for compatibility.
Using 2.5 HDD I can easily get 60MBps transfer if using LAN or 20MBps if using 5ghz
Even if using NAS this will be the same speed that I'm getting. Well, 5ghz speed can double if I install the software on my PC to get 800mbps connection but I'm contend with 20MBps.

To have decent NAS with 4 bays will cost almost 2k
depends on size, 2 HDD might cost 1k to 2k
PC as NAS maybe can save 1k but need to do everything DIY
It's not a cheap thing no matter PC or dedicated NAS.

One thing for sure is at least have RAID-1 on the system if go for PC or dedicated NAS. Having such system means you will have Terabytes of data, although the data might be unimportant or can be downloaded again it will be a hassle and time consuming if one of the HDD spoil. If you not gonna store Terabytes of data then you don't need to get PC or dedicated NAS

My experience flash drive even if USB 3.0 the speed is not great. This is even flash drive directly connected to my PC.

No, on LAN easily can hit 60MBps. To hit 100MBps depends on situation, whether router got download anything, network traffic by other devices, even Windows or router issue also makes it cannot transfer at max speed. Sometimes after restart everything then can hit max speed. Router transfer to PC is easier to hit max speed while PC to router is slower.
I forgot to mention I'm using SAMBA for transfer, you using FTP so that might also give different result.
Also check if enable USB 2.0 mode on the router. That will cause the transfer to be slower.