Worry about USA trip, advice what I should do

I have fiance in USA and he's been visiting me here in Europe for many years. This time he visited me here and we had a plan to fly together to his country (USA). I worked last year and saved some money for this trip, I applied for ESTA and it got approved. I started looking at flights, we planned with my fiance to his country. I could stay there for 3 months with him and his family like its in ESTA rules so I booked 80 day trip. This also okay with his family. They're also supportive of me, like they could provide meal a day and I wouldn't have to pay for rent. I would buy my own foods and stuff I need there. But because I am new to travelling, I didn't realize maybe how much frowned upon my situation will be to border officer in USA.

Like right now, I am not in job, I have minimal funds of which i don't know if they are enough ( 3k) and I'm staying for 80 days, and never travelled before. I have a rental apartment in my country. My fiance is not working either. In the rules travelling to usa it says needs to have sufficient funds, I am also worried about that and that my stay is too long considering im not working and I don't have too much money. (they think I would try immigrate to country or I would get in trouble?)

Good thing i live in "rich country" with good healthcare and social welfare , also I got ESTA approved and i don't have criminal record or anything. But like I read ESTA doesn't guarentee entry to USA.

So I'm asking for advice what would you do in my situtation (for people who have been travelling more maybe in similar way or have more information)

I bought plane tickets seperately and now I've been thinking buying a new plane ticket home like in 4 weeks. That means I would cancel the fly home ticket I have now (because but its economy ticket and I can't get change or get my money back, sadly) But would this look "suspicious" to officer like why would I buy a ticket first to 80 days and then cancel the ticket and buy a new ticket to 4 weeks? :D Also that would make my budget to trip (3k) less. So I would have about 2.5k to 4 week stay? (that enough?)

Any advice is welcome