Another smart step before finalizing your rehab budget is to consult other investors and see how much are they paying for updating each unit.

One contractor told me my 20k-50k budget is not enough.

I would be happy to chat with you if needed - I have a SFH rental in Crestview and my maintenance was way more than initially budgeted for. 

The ability to house hack and not have to pay a housing payment from my personal budget every month.

You should provide budget coverings, at least.  

I'm having trouble sticking to a budget.

If you reduce the nightly price and are very open about the fact that it's not a (5-star) resort, then you'll likely attract a number of people that are looking to visit Hawaii on a budget.

Also budget plays a role as well. 

Guests, employees, food truck, and others will be coming and going during the day.

Just depends on the size and market obviously - but until I verify my budget with a PM, I am usually putting in between $1200 - $1500/unit right now.