I also think if a trader has opened a position already and he chose isolation margin, he will not also be able to convert to cross margin accept when he closes the trade to open new position using cross margin. I hope I am correct about this.
This is going to be exchange specific, for example, I have a feeling bitmex used to allow isolated to cross margin moves (but not backwards).
Try the Fybit platform, I trade there myself, there are no problems.
I'd be careful using a lesser known exchange, at the very least they could have exploitable bugs making them more prone to having funds stolen.
Once upon a time, both Binance and Bitmex were new platforms. I have traded everywhere, and I still continue to use Binance, but Fybit is a good platform in my opinion. It seems to me that this is a former Bitseven, very similar. And Bitseven was much more loyal than all the other major exchanges and they have never been hacked over the years of their existence.