I don’t have a ton of faith in tplink to continue to support omada over the long term. They’ve also been somewhat slow to fix security problems in the past. For the same price as the omada ap you can get unifi u6 lites.
You can still run your own controller and i can vouch thaf a couple of them can cover an entire moderately sized house. I run 2 at home with pfsense on an ewaste tier dell optiplex and have for years without trouble.
I’ve never messed with opnsense but I assume it works just as well.
Also what type of connection are you getting from your ISP? If its a fiber connection you may be able to buy an SFP network card and replace the modem altogether.
I haven’t done any work for the military but i can say that all the legacy systems I’ve worked on were because the specific software they need was written only for Windows 98 and the developer or company that created it is long gone. Keeping it going is a chore but switching to literally anything else is out of the question.
I could see for military applications that having the known quantity of a working piece of software that isn’t changing anymore and can be swapped as an entire unit is an advantage, especially if it doesn’t touch the internet in any capacity. But eventually you run out of people who know what to do if any changes need to be made.