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the best possible way and not to harm my current website
I don't see how that's possible. Right now your new site is doing OK, because Google realises it's essentially the old site on a new address. But if you change that, well, then Google won't look at it like that anymore. All your old link juice will go poof. All your old history will go poof. My advice would be to hold on to it for another year or two, or at least until the link power you get from the domain is miniscule compared to the new one.