So basically we have got a couple of Isteqaras done (one isteqara for each marriage proposal) and they turn out either bad (so that's sure that we don't proceed) or "in- between" (miyana) and so when it's miyana the Sheikh who did the Isteqara said it's better to wait for a proposal where the Isteqara comes out "good" because when it's miyana, it's your decision again and you want Allah(swt) to decide for you. Is this correct to keep waiting for a isteqara to come out good? I actually want the isteqara to be good when i get married, so that I'm sure Allah(swt) thinks this is the right person for me.

So if the istekhara turns out bad, then yes I wouldn’t recommend proceeding.

If turns out miyane, then my recommendation is: if you have done your research about this person, and everything seems good, and this person seems to have the right qualities, then I would have tawakkul on Allah and proceed. But if you have hesitation, or something is making you reluctant, then it may be best not to proceed.