Should I transplant before I start flowering?

One advantage of repotting before flower means a fresh substrate with plenty of juice to see them through flowering. 😎

Relative to that space, there is no reason. "5 gallon" buckets is big enough for 6-7 months of growth, and likely longer than that is fine, too, if in soilless context. You may also be headed toward an over-crowded canopy. Shoot for 2.5 colas per sq ft.. anything over that just translates into more pruning, more wasted growth, more time and resources wasted. If you manage canopy well, you can significantly reduce the work that the peanut gallery thinks is necessary every grow but is really just a bunch of extra work that is 100% avoidable. Should never need to 'defoliate', for example. Maybe, remove a leaf or 2, but never pruning shit off en masse.

If you want bigger plants and have the space... why not?

Yeah definitely, get them settled in there new pots and flip over. Best of luck