I have started that game with the last patch and 128x128 galaxy, just to check the Borg behavior in such game. As I understood there was the fleet support based on Borg population.How is the "formula" for the Borg's fleet support? Does it need to be improved/tweaked?
Early vs mid vs late game.
This game you're playing now, was it started with the new range (12 IIRC) for the Borg Probe? If so, is it ok, or too much?
I suppose no changes for now are needed. I met minor races in a distance of 6, 12 and 16 sectors, maybe the Borg was lucky this time. And after some turns the Dominium started establishing colonies using 1-2 wormholes. The extended range of probes is a must to play the game.
The early game is a challenge, because each fleet of (about) 4 cubes must defend couple of sectors.
The mid game is more or less the same problem because of the expansion. And without that expansion any development of the fleet is not possible. I lost 3 cubes and 3 systems before reaching enough cubes. A lack of cooperation of attacks of other empires is a huge favor for the Borg now, but without that the Borg would probably had no chances.
The AI should also send bigger fleets against the Borg. An attacking fleet of 4 cruisers means nothing for even 1 cube.
The AI are not building any colonies near recognized Borg space.
There are still the same challenges for the Borg in such galaxy - mainly to have enough cubes to protect long borders, especially if a quick expansion is possible. But it took me 600 turns to get sufficient numer of cubes .

I had several 'out of memory' errors between turns 200 and 300, caused by diplomatic actions of other empires in majority. Now I can play 20-30 turn with no problem.
I liked that game, despite monotonous construction part in Borg colonies. It was very tactical, permanently to chose what systems to attack and when.