The reasonable volume expectations of the WA12 running 300w into the single 4ohm combo cabinet.
If your WA12 is a 112 combo, you don't have enough rig for the gig. I use an Eden EX-112 (handles 300-watts rms) for small venues and only use a 225-watt amp (Eden TN-226) with it because that's all that's needed for small venues. I used two of them with an 800-watt Eden amp (400-watts per channel into 4Ω or bridged at 800-watts into 8Ω) for medium gigs. Could probably have used that same set up for large gigs too, but chose to use my DNS-410 cab for large venues, outdoor venues, and music festivals.
After I got accepted to the DNA Endorsing artist group in 2019, I bought a DNS-210 cab that handles 700-watts rms and can use that for all my indoor gigs. I drive it with one of my 800-watt amps and it's an exceptionally loud and clear cab, so I don't need to roll in my 410 cab. Plus it has all the sonic characteristics of my DNS-410 cab, which I love. I still usually take that smaller Eden set-up to small venues like brewpubs though because it sounds really good and I can carry amp, cab, bass, and gig bag in or out in one trip. I sit it on a chair. For indoor medium and large venues I bring along my my DNS-210 cab along with an amp stand, which holds it up nicely off the floor and tilts it back at 45° making it easier to hear on stage.
I don't use any of that smaller gear for outdoor venues. Outdoor venues, since they are not entirely enclosed, don't have nearly the amount of reflective surfaces to bounce your sound around inside of. So they require a lot more power with more speakers. I always use my DNS-410 cab outdoors. Since you were getting no help from the FOH at that gig, you really needed to have an amp and cab that could fill that open space - a 112 is not the answer to that challenge. At the very least you'd need an additional 112 cab to give you a 212 set-up and that might not be enough in large open areas. But with a 212 setup, you can usually at least hear yourself on stage
Got a question for your on your WA12 combo. Is that a Warwick combo? I see Warwick cabs rated at 300-watts handling power, but don't see any Warwick 112 combos that are anywhere close to being 300-watts, 112 combo. Also, most combos (not all) list the power that they can put out with a 4Ω load and you have to plug in another 8Ω cab to get that full power. Using industry standard parallel connections, running two 8Ω cabs puts a 4Ω load on the amp allowing it to access its full rated power.
A 300-watt into 4Ω amp would only put out about 150 to perhaps 200-watts rms using its internal speaker. You have to add another speaker to get full power out of it. There are a few companies like Eden who use a 4Ω speaker in their combo to get full power as-is. However, if you hook up another cab to it, it disables the internal speaker and only uses the one you hooked up - not very verstile.
As you already heard, a sub-woofer is not going to give you much help over most of the frequencies you play. Typically they handle 200Hz or less, down to about 20Hz and you play a LOT of frequencies above that. It might help a little but it's not going to make the difference when playing outdoors. As far as hearing your bass goes, if the bass is not in the PA or FOH, it's not going to be in the monitors, and the monitors won't help you hear yourself. You'll hear the rest of the band fine, but not your bass. Usually a sound technician that will not put the bass in the monitors or the FOH system excludes the bass because they are worried about the bass frequencies damaging their speakers.
So, if you expect to do a number of outdoor gigs, you'll be best served by at least getting an extension cab to plug into your combo (presuming it does that) and I'd honestly suggest you up your amp power to ~500-watts or more rms into 4Ω. 300-watts would be fine for small and medium venues and a 500-watt into 4Ω amp will give you somewhere between a minimum of 250-watts up to a bit over 300-watts. But for large and outdoor venues, you're much better off with at least 500-watts and at least a 2x112 set-up. A 410 set-up is better yet.