Hi Florix! Welcome to Fishlore!! I can't help much, but I can answer a couple of questions!
First, be patient during the cycling process. Anything you may do to speed it up may come to bite you in the future. Let it go and just wish it to be faster.

Did you add a raw shrimp or anything to get the cycle going? Have you noticed the ammonia,
nitrite, Nitrate spikes happening??
As for the QT...I know I may get blasted on this one...I don't have one. I buy all of my fish from the same
LFS and I have a good relationship with them. They stand behind their product if something happens. So absolute necessity...no. But totally up to you. Most people will say YES!
Switching to a reef tank: You'll probably need to upgrade your lighting, add some live rock and a few other things. So, it'll be pricey, but you can upgrade slowly over time. Adding live rock here and there will help make the cost less painful (make sure it's cured!!) It's only a pain if you don't enjoy the hobby. If you love the hobby...it'll be constantly making fun improvements. It depends on how you look at it.
Enjoy your new tank!!