The cost of the agricultural feedstocks to biofuels are not easy to reduce further, whereas renewable electricity is getting less expensive and likely has much further to go. I think CO2 to fuel will have a decisive cost advantage over biofuels.
The reason to use direct air capture rather that going for higher concentration point sources is one of scale. We want to produce fuel at scales much larger than can be accommodated by any other means that capture from the air.
Why not go for the higher concentrations now? Why not target agricultural ethanol plants now? I grant that long term they are limited, but why not use them now?
Higher CO2 concentration seems like it ought to make your initial testing easier.
Likewise, if you are really more cost effective than distillation all the existing ethanol plants would love to replace their boilers with your system. It seems like a simple way to get a small cash infusion selling to them. They can convert their ethanol to gasoline as well, and have infrastructure in place.
What I'm saying is don't bite off more than you can chew. If any piece works now push it while getting the others working.
The reason to use direct air capture rather that going for higher concentration point sources is one of scale. We want to produce fuel at scales much larger than can be accommodated by any other means that capture from the air.