So many devices are eating up your Internet feed so this is NOT a hardware issue. As to 2.4 and 5GHz you use what's faster in your case. No one should rule that one is better. We test that out.
The 500Mbps is only when you are the only user. Which you are not.
Any Windows 10? Disable this NOW: https://www.howtogeek.com/224981/how-to-stop-windows-10-from-uploading-updates-to-other-pcs-over-the-internet/
I am a novice at this so please use layman terms. I have fiber optics. They only guarantee 500mbps if you are wired into the dual router. We have a lot of wireless devices. We have 2 iPhones, 2 iPads, 2 laptops, 2 Samsung smart TV’s & a wireless printer. It is worse when we have visitors that connect to our network. Range doesn’t seem to be a problem. I can sit right next to the router & have a very slow connection. Is there any hardware in existence that can boost a wireless signal & make it faster? I also get conflicting advice on using the 2.4 & 5 connection. Can anyone give me a difinitive answer about that?