Yeah, so Luffy has effectively used all his gears during this fight. It seems as if he’s switching between them to mitigate haki drainage, but Luffy’s definitely not keeping up just by being in base. He’s being strategic
When luffy use gear fourth, luffy began look different person literally. look like it is transforming like SSj form from dbz. Doubt luffy switch base to gear fourth in second like goku switch super saiyan god to blue in second when battle hit. (Dragon ball super manga)
What we see base luffy use gear2/3 on hybrid kaido . We has not seen luffy pull gear fourth move while staying base luffy.
When luffy pull gear fourth, his body begin to change different than normal base luffy.
Next arcs,I think luffy will master gear fourth and can keep as unlimited time or even longer time,