I presume that you have already worked with the ISP and are convinced that it isn't the DSL line or the equipment on their side. If not, I would start THERE. You will spend a bunch of time on the phone, but if the problem is ISP related it will take a while to get fixed, so you should start now. It will likely take a bunch of outages for them to admit the problem, because they close problems if an equipment reset fixes it.
I would obtain (buy or badger the ISP) a replacement DSL modem and see if that
fixes the problem. If it does, you're done. If it doesn't, reset the DSL modem to factory settings and return it, because the problem is the WatchGuard.
If you need to replace the WatchGuard (and it isn't under warranty or service), I recommend Astaro. $1000 will get you a great firewall appliance for a small office.
You can also test the WatchGuard directly. If you can put up with some internet downtime, replace it with a standalone PC. Or if your firewall config is simple, get a free trial from Astaro. If the connection remains alive long enough (say three or four times the typical time to failure) then you know the WatchGuard was the problem.
Note #1: You don't need to do these serially, you can move forward on all three at the same time.
Note #2: You can build a great firewall on the cheap with a PC with multiple NICs and something like FS-Security or SmoothWall, but I wouldn't unless you are into rolling your own stuff or are horribly strapped for cash.
For the first project for the wireless link you can get away with a regular wireless router (Linksys is a popular choice for such applications), directional antennas and weather proof housing powered via Ethernet cable. People reported stable links up to 40km/25mi using this setup with good directional antennas, so 500m would be piece of cake for this setup. Here is an example of what I'm talking about.
For your second project I would recommend one of Juniper firewall appliances - either SSG-20 with an external switch or SSG-140. The software on these appliance is of professional grade, so configuration can be as flexible as you can imagine it and they priced for SOHO/SMB.
Obviously there are plenty of other options but this is what I had hands-on experience with and what worked best on several of my projects.
UPDATE: It seems Juniper is phasing out their SSG line replacing it with SRX, so if you planing longterm then SRX is the way to go.
Best Answer
The MikroTik RouteOS is very capable, but definitely designed for smaller installations. They're really aiming at hot-spots and small multi-tenant buildings. FreeBSD is going to be a significantly more capable, configurable, and able to provide greater capacity.
What exact pain points are you experiencing with your FreeBSD servers? You say you want to "enhance the administration of the server", but that really doesn't mean anything.