Mythical is better to upg to asap for example overture talisman vs the new higher skywatch talisman. If you fail say level 5 in the new higher skywatch talisman you get knocked down 2 levels (from 5 to 3) where as in overture only one level (from 5 to 4) so it's more punishing when you fail. Where you won't have a choice is Acc's because they are swapped out for newer acc's. With Soul, Pet, Heart, and Talis though it's pretty much better to if you're going to mythical do it as soon as it's eligible rather than up tier a whole lot and have a more punishing level reduction fail system. Be advised though if you do it to a weapon you can no longer go PVE path and take advantage of like reduced mats costs and swap back over.
Skywatch Talisman (Also Uzume Ear/Ring and Spirit Bracelets)
Everything else but the new Skywatch Talisman, Uzume Earring, Uzume Ring, Storm Spirit Bracelet, Wild Spirit Bracelet, and Thunder Spirit Bracelet.
Success and cost is retained what isn't is how many levels you drop when you fail. So it isn't "even" to say use 10 Enchantment stones later when sure everything is basically equal with the exception of punishment upon level failure. I'd honestly say upgrade it now or asap because it's more discouraging losting 2 levels at a time during low levels than just 1, gonna def want to save up like 300 fusion stones when you make an attempt to +11 just incase you aren't lucky at all. There's nothing to compensate for losing more levels later such as better rng or cost reduction. In acc's it wont matter as they are crux'd and moved on so you have to start over and deal with a worse punishment system if you fail. For things like Your Soul, Heart, Pet, and Talis you can do something before you get too high. What's the next tier going to be -3 levels early on? Better to do weapon earlier too before a higher weapon is likely to reflect the same kind of loss punishment levels in the future. Though you would be unable to go PVE path to circumvent cost once you mythical a weapon.