Onward ticket dilemma, please help.

I plan on travelling to a country and staying for the whole 3 month that the visa allows and would like to have the option to pick which country I go to next or extend my visa in said country.

I’m sure you guys all know that most countries need proof of an onward flight leaving the country, and that’s my exact problem because it strips me of the freedom to choose later.

I’ve heard of onwardflights.com and I’m not too sure if I should try that because I’ve heard some people have horror stories at immigration.

I’m not an American so fully refundable flights come with a fee that gives you the right to cancel-> get refund. It’s usually a couple hundred extra.

What should I do?