I haven’t found a way to improve the payout on the blooms, unfortunately.

What I’ve learned to do is two things:

• Attack mushrooms in groups (when possible). The higher the level the better, usually.
• Feed scarce nectar to immature pikmin who have buds on their heads - this pays double.

Sometimes, I attack mushrooms at less than 3 or 4 stars, so I can get more, different types of nectar, instead of a fewer, bigger payouts.

One other thing that helps:
If you can walk with other players, you need fewer flowers (personally) to force the bloom. Either way, the payout, I think, is the same. So walking partners help.

I walk, most days, with my wife, who also plays.

Editing to add:
I’ve found they will usually pay out the type of flower that was planted most. So if 151 sunflowers were planted, it’s likely paying out sunflowers. If 151 roses are planted, and within all the flowers planted blue was the most numerous, you’ll likely get blue roses.

I have found, however, that this doesn’t always work. My wife and I have walked through a bud which had 0 planted flowers and we planted it to 300, forcing the bloom. We’d exclusively planted white hibiscus. We got yellow. Then on the next, we got red. Some scarcity, Niantic appears to force or manufacture, unfortunately. When I saw that, that’s when I started doing the bud-pikmin double-up feeding much more consistently.

I found this and I think it’s a significant improvement over anything I said earlier.

Now what I said, I think, is accurate, but the link goes into greater depth on how things will sometimes act differently than you’d expect.