Subs generally don’t work as well outdoors in an open environment as they do indoors.  I think you would have to try it yourself to determine whether you think it makes a difference or not.

More importantly, the speakers use WiFi to communicate and WiFI signals are interrupted by human bodies.  There are ‘horror stories’ of weddings where the Sonos setup was great in testing before all the guests arrived, and filled with dropouts during the actual event.  You might want to consider a temporary wired solution for your event instead.

You can try repositioning your Sub and see where it gives the most sound, in a corner will give the most, on a flat wall helps some and free standing gives the least audible output.

Long Ethernet cables are cheap, I’d sure spend the money for at least the Fives. Possibly one long cable, a cheap switch and two shorter cables if that works out cheaper.