HDD speed also matters. Did you buy 5400rpm disks?

Another issue is network cable. Get at least CAT6 cables. Even CAT7 cables are cheap these days. You can get full 1Gbps speed.

You're accessing it outside of local network? If in local network is it fast enough? If local network also not fast enough then likely the NAS is the problem. I see it only have 512MB RAM and no m2 slot for cache. One way to speed up the data access is change to use SSD instead of HDD.

If local network not slow but over the internet only slow, then need to check a lot of things. 100mbps plan IINM is 50mbps upload which is good enough but you behind CGNAT, so likely you're using Synology QuickConnect and data relayed over their server, they also capped the speed to 30mbps IINM. You can use Tailscale instead for the connection, based on my test for a month or so it give speed almost as good as direct connection with port forwarding.

DS224+ will be better than your current NAS as it have faster CPU and more RAM. But if you going to upgrade your NAS I'll suggest instead get DS423+ as it have 4 bays and 2 m2 slots or DS723+ as it have faster CPU and also 2 m2 slots.

Do you have few TBs of data in your NAS? If not much, the cheaper way is to try by using SSD first to see the improvement is good enough.

500mbps upload speed is only 100mbps, double than your current speed. For photos it won't make much difference. If your video size is by hundreds MBs then will see improvement but even then will not be fast enough if the video file is large. I just tried download 178MB file from my Synology, my upload speed is 100mbps with port forward and my phone on 4G connection, it take 2 minutes 52 seconds to complete the download. My suggestion is better to stick with current package but use Tailscale and also re encode if videos size is big.