It seems like a good moment to get into the market now (although I'm already exposed to appreciation through my residential property I would like to add another one as a rental).I found a partner with money and a lot of real estate experience in the UK that is willing to invest in The Netherlands with me.

Not sure, because at that point you're speculating appreciation, which is more of a gamble than investment.Refinancing to break even and using the cash to by a cash flow property may make sense.

If your history shows that you haven't used it correctly in the past, why should they take a gamble on you now?

If you're purely speculating on appreciation, there are better cities to gamble your money on.

(I lived in two duplexes and turned one of my residences into a duplex as I was building my portfolio and those were mostly 1% deals)  If however you think the appreciation on that property will be far faster/higher than others expect then it might be worth a leaner deal at 1%, but appreciation plays are mostly speculation/gambling.  

BLINKERS OFF GUYS  It sounds like your earnings are gambling earnings so I would strongly recommend opening up an llc or Corp and pay yourself a k-1 income.  

that's for him to gamble on.

If you do not have those and still buy, you are gambling ... you may win, you may lose, but keep doing it and eventually the odds will catch up to you.

I am new to the US rental market; but had been a landlord in the UK for 15 or so years.

Nobody knows the future, but I wouldn't gamble on price growth in 2023.