2010 yamaha 4 stroke 8hp prop change suggestions?

Here are my basics:

Yamaha 2010 Four Stroke 8hp
-8.5" diameter and 7.5" pitch
Tracker 1436 (stock) (flat bottom)

I am looking to increase top speed. Right now, when it is just me in the boat, I cruise at about 15 knots according to the Navionics app on my phone. If I have another adult in the boat on the front seat, the boat feels like a slug on the mid range, and tops out at about 11 knots.

Where I am, I need to make a little bit of a trip to do any fishing or get anywhere good. I have a pretty good grasp on how pitch affects rpm and speed.

I am looking for advice about a possible prop change to 8.5" pitch (or more?) and what that would do for me and my current setup. I don't need the low end as long as I can still get it planed off. Is an extra inch of pitch too much? Not enough? Mostly it will be just me, but 60/40 I will have another adult with me.

Again, I get the basics on the rpm change with prop pitch, but will I improve gas mileage with greater pitch if I keep it planed off? I guess this could be a benefit? With a 8hp, I don't really think I am worried about mileage, but just wondering.

I guess I should also mention that I have no way of knowing my actual rpm at any given time. Just working with what I have, and trying to make the most of it.
Thanks to anyone who can help or give advice!

What is your maximum RPM currently?

I have no way to tell rpm. Unless somebody knows a cool trick to find out without a gauge.

It is impossible to know the RPM without a tach. And thus, it is impossible to make a prop recommendation without knowing the current RPM. Usually a cheap inductive tachometer can be had for around $30-$50 and it is not a permanently mounted tach, if that is appealing. It can be permanently mounted but doesn't have to be.

https://www.ebay.com/itm/TT2A-Hour-Meter-by-Tiny-Tach-Reset-able-Job-Timer-MADE-IN-THE-USA-/261062865923?hash=item3cc88f3403:g:eXsAAOSwvUlWsMoU