Just because there were the same number of carriers didn't mean that they were equal in strength.
For a truly professional fleet to unleash the combat abilities of a battleship would require at least ten years of training and operation.
The technology on galactic battleships was more developed, but it was also more complicated to operate them.
The reason why Luke had chosen a 'once-and-for-all takedown' was because Earth's allied forces didn't have the ability to fight Thanos's army in a proper galactic war.
He had no other options.
Thankfully, this was still within Luke's worst expectations, and wouldn't affect his plan to kill Thanos.
When Luke let the core members know, nobody was scared, not even Dr. Banner, who had been beaten up by Thanos.
That was because the trump cards which Luke had uncovered in the last few days were truly unimaginable.
Skye and D.A. Jennifer, in particular, completely shocked many superheroes.