Are you the owner of the business, or is the business owner paying you for professional IT/network support? I'll assume the former out of courtesy.
Your ISP provides a 500Mbps link. Have a read of So you have 500Mbps-1Gbps fiber and need a router READ THIS FIRST for some ideas about likely hardware requirements to support a connection that fast.
You want WiFi. Consider standalone access points for wireless networking, rather than all-in-one devices which necessarily compromise on performance to meet a price.
You want WPA3 Enterprise. That's likely to involve RADIUS and certificates and possibly more, and can be challenging to implement. It's not impossible for a beginner, but be aware of a shallow learning curve: it'll take time to acquire the necessary knowledge to do it well. That'll be some fun bedtime reading for you.
You want parental control. If you're the owner of the business, this could be met by talking to your employees. If you don't trust them, there are other problems here. However, if you still require a technical solution, one option could be a service such as OpenDNS. OpenDNS offers the capability to permit or block categories. Using it requires all your DNS to be funneled to OpenDNS and blocks for all other DNS providers (otherwise it's easy to circumvent). OpenDNS offers both free and paid-for services. There are other alternatives; OpenDNS is merely one option.
You want a strong firewall. OpenWRT uses iptables, which is a proven robust firewall. However, it's layer 3-only (to the best of my knowledge; if I'm wrong hopefully another forum member may correct me). In other words, it controls traffic based on the IP address and port number. For greater filtering based on actual traffic content, you'd need a layer 7 firewall which does content inspection. And those tend to be expensive to buy and usually have an associated subscripton as well.
You want antivirus. Windows Defender is included with Windows. I believe there are some free alternatives (can't recommend any as I don't know them from experience), but pretty much all antivirus these days is paid-for, on a subscription basis. I'm unaware of any antivirus which is sold on a pay-once model.
You want one-off payments or free, not subscriptions. How much is your time worth? Learning new stuff is great, and worthwhile, and I'd always recommend it. However, be aware of how much time you may have to devote to learning something you're not familiar with. If you're also trying to run a business, you might find it very challenging to do both at the same time.
You want open-source to avoid vendor backdoors. If you have good enough skills to audit source code for vulnerabilities, have you considered working as a security auditor instead?
From a cursory read of the Endian website, it appears that Endian offers content inspection, which a layer 3 firewall does not. In addition, it looks like Endian is very expensive and geared towards larger businesses. That latter assumption is a guess based on the coy "contact us for pricing details" button instead of an up-front breakdown of their prices. That sort of behaviour is common among vendors who sell into large enterprises. Of course, I could be wrong and Endian could be affordable. But if so, why not use that as a selling point?
Advice on a high-level setup? OpenWRT could be a good starting point, but it's unlikely to be sufficient on its own to achieve your moon-on-a-stick requirements. My notes and links above should give you enough keywords to plug into your favourite search engine for further reading on your educational journey.