In short, people are stupid and shortsighted. Most of the problems in that blog rant are either unsolvable, irrelevant or "thinking is hard". I have had working IPv6 for nearly a decade - using a HE.net tunnel, because most ISPs won't provision IPv6 in Australia. And I'm by no means a network engineer.
Same here, I've had IPv6 in my home network (in multiple variations) for years and it works well and solves actual problems.
Back to the OP's question: with the configuration you have, you'll need to setup IPv6 address configuration and routing at each hop, you can't expect RAs to *pass through* layer 3 network elements (routers). RAs are *local* configuration, they don't pass through routers.
Assuming that FIOS allows you to use DHCPv6-PD to get a prefix delegation of reasonable size (/56 or larger), which they don't yet in my FIOS neighborhood, you'll need to do various things:
* The OPNsense firewall will need to obtain a delegation from FIOS using DHCPv6-PD. It may also get its own address (via SLAAC or DHCPv6) but that is not mandatory, as it will already have a link-local address that can be used for IPv6 transit between itself and the FIOS network. The delegation will need to be at least a /56 in size, although a /48 would be better given your configuration; if it's a /64, you're stuck because that can't be broken down further.
* The 7250 'core router' will need to be given a block of addresses it can use on the VLANs it manages. Just like the FIOS-OPNsense link there is no need for addresses to be assigned to the transit LAN, that LAN can use the automatic link-local addresses. The block can be given to the 7250 via DHCPv6-PD *from OPNsense* (not FIOS) if OPNsense is able to do that; if not, it can be manually configured on both ends.
* Each of the VLANs on the 7250 will need to be manually configured with a sub-block out of the 7250's block (but no smaller than /64). The 7250 would then be configured to emit RAs on the LANs so that hosts can get IPv6 addresses and have routing information. It can also provide DHCPv6 services if you desire, but that is not necessary.
* There is no need for any dynamic routing protocols for this to work; the subnet addresses form a 'tree' starting from the block delegated by FIOS, and each router in the tree knows about the downstream routers, but there's no need for them to know about 2nd or 3rd level routers/routes below them.
* "passing through" DHCPv6 traffic via the OPNsense firewall isn't likely to work out well. That sort of thing can work OK for host addresses (not prefix delegation) if the device doing the passthrough supports NDP proxying (similar to ARP proxying in IPv4), but that isn't going to work with DHCPv6-PD.