So yes, if you did get a Mythic piece, it would churn out a lot of “saved” upgrades for you overall, especially in place of a champion piece or lower. However, is also sounds like you aren’t getting much opportunity for or having rough luck on getting mythic pieces.
Also sounds like you are doing just fine in racking up crests. Given that you won’t be asked by the game to re-spend crests (valorstones are entirely different, but you are probably having no issues there either), I honestly feel that any hero or champion piece is game for upgrades. Holding out for the mythic piece might feel good when and if it pays off, but depending on the size of that if… usually better to press with the upgrade.