The aircover problem would of course be in single engine aircraft, without drop tanks. Fighter cover from the long range P38, P47, & P51 types (nd Typhoons?) would have been more than adequate. The twin engined bombers had routinely operated at greater ranges during the previous year of preperation over France & Belgium.
To put the airpower problem in perspective: on 6th June the Allies were able to fly over 14,000 sorties over France. The GAF with less than 285 operable aircraft in France/Belgium was able to start a little over 300 sorties, of wich less than half reached the enemy. During the first 30 days of Operation Overlord the German AF peaked out briefly at some 1000 sorties launched in a day, with the average down below 300. The Allied AF never fell below 10,000 daily sorties for the first thirty days, including bomber raids on Germany.
A second problem for the GAF was the airfields near the Breton battlefield were high risk. Any aircraft deployed closer than eastern or southern France would have been subject to regular suppresive raids on the airfields.
Bottom line for the air battle is, in the opening weeks the Allies would have a few thousand fewer sorties over the battle. Maybe as few as half the norm over Normandy during the first thirty days. That still would have outnumbered the GAF by a factor of 10-1 or better on most days. Of course many other Allied sorties would have still be flown against the German airfields and LoC deep in France. Only a few thousand very short ranged aircraft, like the Spitfires would have been excluded from the battle in the first weeks. After three to four weeks local airfields in Brittany would have allowed the deployment of the short legged aircraft.
Logistics would have been more difficult for the Allies at first. Op. Overlord depended on the large scale use of small coastal freighters and barges. Far fewer of those could have been used supporting a Breton invasion. Once Normandy was cleared and Cherbourg secured then the more of the coasters could have been used. On the up side Brittany had a larger potiential port capacity than Normandy. It is largely forgotten now, but the Allies had intended to build a third prefabricated port in Quiberon Bay. There are multiple reasons why that operation was not executed, but with a Breaton invasion it would certainly have been. Between the Quiberon Bay site, and the rest of the Brittiany port group the Allied supply capacity would have reached some 35,000 -40,000 tons per day by D+60. Or over one million tons per month. About 30% better than the best results of the Mulberrys/Cherbourg combination on D+60. So while the coastal freighters would be sorely missed at first the ability to take in more deepwater ships direct from the US would help reduce the sting after some 60 days.