@Singletrackmind, the Boa closure requirement narrows the field quite a bit for flat pedal shoes, since most of the stalwarts in this category have stuck with laces or velcro. But there are some solid options worth looking at.
The shoes that specifically combine flat pedal soles with Boa closures tend to come from a smaller pool of manufacturers.
Ride Concepts is probably your best bet here. Their
Tallac model uses a Boa dial and has earned genuinely strong reviews for comfort and durability. The sole compound isn't quite Stealth Rubber levels of grip, but community feedback suggests it's more than adequate, and several members here rate Ride Concepts very highly for comfort over Five Ten. The fit does run narrow though, so worth checking sizing carefully.
Etnies Camber Mid Boa is another option that's been floating around the gravity shoe market. Michelin rubber sole, Boa closure, mid-top protection. It's aimed more at the dirt jump and bike park crowd but works perfectly well on trail. The sole compound is decent without being exceptional.
Specialized 2FO Roost (the flat pedal version with Boa) has also been in the mix, and given you're in San Diego you'd have easy access to Specialized dealers for fit checks. One of our members noted that Specialized shoes come in at around 90% of Five Ten quality for significantly less money, which tracks with what I've seen elsewhere.
Now, regarding the Five Ten Freerider, you're right that it remains the benchmark for flat pedal grip thanks to that Stealth S1 rubber. But as you've noticed, the colour options are limited and crucially, the standard Freerider doesn't come with Boa. The Freerider Pro and Pro Mid use laces or velcro straps respectively. Community opinion on the velcro mid-tops is mixed; several members have noted the velcro loses its stiction after a few months, long before the soles wear out. One member specifically called out that the straps will rip if you pull too hard.
Given your San Diego riding and the dry conditions, breathability matters more than waterproofing, which opens things up. I'd point you toward the Ride Concepts Tallac with Boa as the strongest all-round option that ticks your specific boxes. If you can find them to try on locally, that narrow fit question is the main thing to sort out.
I've also got community data on pedal and shoe pairings if you want to optimise the combination for your setup.