At which point you might as well save yourself some dosh, and get say a Ubiquiti EdgeRouter 4 or equivalent Mikrotik for less cash that is fanless and lower power and for which support does not run out in three years time rendering the device useless.
I use EdgeRouters but they don't have the transparent drop-in capability of WatchGuards so would require a reconfiguration of the setup - something which the OP wanted to avoid. Now it seems that bridge mode is possible, that's not an issue.
I'm not sure what you mean by 'useless'. All basic security functions of a WatchGuard continue to work after the licence expires - including things like application proxies, NAT, WAN load balancing etc. As a security device they're still more sophisticated than any of the Ubiquiti devices - even with an expired licence.
What you do lose is the ability to update in the event of a significant security bug or to add new features. Things like gatweay antivirus, geolocation blocking and spam filtering require an additional subscription beyond the basic support package so don't come as standard on the lowest priced support options anyway.
I would avoid Mikrotik completely given the poor history of security, the results of which I have experienced first hand
. When I posted that I'd identified four compromised Mikrotik routers, there have been quite a few more since. Just looking at today's firewall logs I see this
has been compromised and is probing my connections. I shall be reporting this to the ISP later.