There is another way to get untagged Mythics from Diablo 4's Horadric Cube
Some Cube recipes can randomly create Mythics without giving them the usual Crafted tag.
We already have a way around the Diablo 4 Season 15 Crafted Mythic restriction by using the Horadric Cube's recycling option, but there is another way to get around it that works a little differently. Mythics that are created randomly through other Horadric Cube recipes do not receive that tag either.
And that opens up a pretty interesting alternative.
You Can Gamble With White Items
One of the Horadric Cube's recipes allows you to turn a basic white item into a Unique. Most of the time you will just get a random Unique, but there is also a small chance that it can produce a Mythic Unique instead. The downside is that you have basically no control over the result. Unlike the normal Mythic crafting recipe, you are not upgrading a specific Unique and keeping its stats. The Mythic you get is randomly generated, so you could end up with something completely useless for your build.
But if you get lucky and the Mythic has the stats and power you need, it will not have the Crafted tag.
That means you can equip it on top of another Mythic made through the Cube's regular Unique-to-Mythic recipe. So the recycling trick is still useful if you are willing to reroll your Mythic's stats, but this one is a good way to get other untagged Mythics. And worst case, you can always get a few 
Resplendent Spark from it.
It Is Not Just the White Item Recipe
The same interaction can apparently happen with other Horadric Cube recipes that have a small chance of producing a Mythic. For example, you can put boss trophies into the Cube through the Amalgamation recipe, because this can also randomly produce a Mythic without giving it the Crafted tag.
That means the white-item recipe is not necessarily the important part here. What you need to remember is that random Mythic outcomes from Cube recipes can bypass the Crafted restriction altogether.
I do not know if this is an intentional interaction or simply an overlooked consequence of how the recipes work, but if you saved up a lot of white items, and you have the materials needed to turn them into Uniques, now might be the time to try your luck. Apparently, some people get quite a good amount from that, and worst case, it is an additional source for Resplendent Spark as well.