Arguably this stacking of the Supreme Court could have been prevented if Justices had retired when the Democrats still had control of appointing their replacements

No, nothing would have changed if that happened. Republicans have no qualms about overtly breaking the law and abandoning their duty and decorum. If they did then Garland would be a sitting SCJ and Gorsuch wouldn't.

If RBG had retired while Obama was still president, it would have been a Democrat appointment, no?

Am I missing something?

yes. Scalia died in Feb 2016 (while Obama was president) and the senate refused to hold confirmation hearings on a successor until Trump took office in 2021.