What is the relationship between mileage and tyre's size

My bike has took me some 25,000 kms. I have seen many cuts in my vehicle's rear tyre. I was suggested to change it due to regular punctures.
Unfortunately, manufacturer did not give tubeless ones. Now, I want to install tubeless ones seeing rainy season in Mumbai! 😁

I want suggestions on:-
1. I have seen some different tyre sizes. Does it affect the mileage?
2. I have heard, tubeless tyres are best. They can retain even if puncture happened for a significant amount of time (as compared to its tube's counterpart.) True?
3. For installing tubeless, is the showroom best place to do? or some local shops. (Please let me know if anyone done this)
4. If anyone is aware of the price of tubeless tyres for Pulser 150, please let me know? (Yeah, I have seen online, but I want the price quote from a guy who did this.)

Thanks in advance.