If those arenas are isolated, your fleet either can't enter them or can't leave them. Which one are you suggesting!
Can't enter, that's the whole point. Obviously, you would move the fleet out of those arenas before willfully isolating them. That forces the Reapers to come to you, via FTL, over a period of months. Meanwhile, the Reapers which remain in the relay network can be dealt with in a more manageable manner. This ensures:
1) The Reapers massive numbers are rendered moot.
2) The Reapers that remain in the relay network are cut off from large numbers of reinforcements.
My point is that even within the isolated clusters you've got no real ability to bring the Reapers to battle.
I think you may have misunderstood me. I am not advocating fighting within the isolated clusters. I'm advocating abandoning them, and all the Reapers within them, while continuing to fight within the relay network. The Reapers within the clusters will finish their slaughter, and then rejoin the battle by FTL - which will take months, and possibly even years for them to do. Meanwhile, they are effectively removed from the rest of the fight. I'm not sure what is so hard to understand about that. As I said, it is basically the same strategy as that employed at Thermopylae, except on a galactic scale, and it is at its core a fundamentally simple strategy.