Even for a 2 week trip I went loaded with employment contract, lease agreement, bank statement, etc. I was asked one time who I was visiting (fiancé at time) and then about my employment. I was let in but if I wasn't able to evidence strong ties to my home country, it may have been another story.
You really need to show a reason to return. If it's not a job, what else is there? Do you rent or own property? Do you have any pets registered at a local vet? Any big events, like a wedding, you're invited to attend? Some things might seem weak, but it's better than nothing. You kind of need to go assuming you will be refused, so how can you convince them otherwise?
No hard rule on money. I was never asked about it, but for 2 weeks I'd take about 600 USD cash and have some credit cards. I believe it can look suspect if you try to enter with a large amount of money. It might look like you are going to be funding a life, not tourism.
Youve already done a fairly long trip, so if it were me, especially if you are engaged with K1 or spousal on the horizon, I would be changing it to a shorter trip. Either 30 days again, or less. Also a very good point about your partner going to EU to see you instead.