***** Solution *****
Okay, after reading through the pfsense documentation more thoroughly, and exercising some patience to let Cisco/Merkai establish the correct links, I have a working setup.
As stated previously, pfsense randomizes ports for security/stability reasons. This is something that regular consumer-grade routers don't do, apparently. Per the pfsense documentation here:
By default, pfSense software rewrites the source port on all outgoing connections except for UDP port 500 (IKE for IPsec VPN traffic). Some operating systems do a poor job of source port randomization, if they do it at all. This makes IP address spoofing easier and makes it possible to fingerprint hosts behind the firewall from their outbound traffic. Rewriting the source port eliminates these potential (but unlikely) security vulnerabilities. Outbound NAT rules, including the automatic rules, will show fa-random in the Static Port column on rules set to randomize the source port.
Per Cisco documentation (and my IT team), I gleaned that the Meraki doesn't like the source ports to be changed.
So to fix this, you need to create an outbound NAT rule for the Meraki device. Go to Firewall->NAT->Outbound.
Select "Hybrid outbound NAT" as the mode. Create a new rule/mapping as follows...
Interface: WAN
Address family: IP4+6
Source: your internal subnet (I just targeted mine to the /32 that my Meraki is assigned)
Source port: (blank)
Destination: Any
Dest port: (blank)
Static Port: be sure to check this box!
I suppose you can whittle this down to the specific Merkai ports (7351, 9350-9351), but this is a single-purpose VPN device and I just figured I'd avoid future problems by just setting this to any port. Same for the destination...you could probably set this to your company's Meraki IP, but again, this is a security device (router/firewall/VPN) that only talks to the Cisco cloud and to the VPN concentrator, so that should be unnecessary. Here is what the rule should look like.
ea72fad1-5242-42ca-8fd7-22adfb57e02c-image.png
Again, my company uses Auto NAT traversal and has our Merakis in site-to site mode and this worked for me. If they used manual NAT traversal, then you'd probably have to set a couple of different rules mapping the home Meraki to the company concentrator.