Some things I've noticed with Mythics.
1) Probably too easy to reconstruct initially
2) Cannot deconstruct to get transmutes back
3) There is no incentive to use antiq more than once, there should at least be some benefit to completing the codec.
I think it would probably be ideal to treat the codecs in a similar fashion to how collecting more set items = cheaper cost. Collect 5 codecs = 50, 10 codecs = 25, all 15 = free. If the goal is to get the cost of Transmuting the item to 0, it does not matter that it cannot be deconstructed. This would incentivise people to collect all 15 codecs. Another thing that could happen from this is at 15 codecs, the item is deemed collected and the leads no longer drop.
If that was in place it the three points would become
1) Initially quite costly to reconstruct compare to other items, but codecs can reduce the reconstruction cost to 0.
2) Cannot deconstruct, but doesn't matter as the most you'll get back is 0 because the cheapest cost of reconstruction is 0.
3) You are incentivised to collect all 15 codecs to reduce the reconstruction cost to 0.
4) No longer spammed with Mythic leads after completing all 15. The Mythic is deemed completed. Fully researched and understood, therefore reconstruction should become easier, the best minds in Tamriel have been studying what you've found. It also doesn't make sense that such a unique item can be found so many times. It is unique, one of a kind (at least to you, being account bound). They shouldn't rain from the sky, there should be some sort of finality to their procurement.
I'll add. It is very unlikely that a player that has access to all the required lead locations doesn't also have access to a transmute station via either CWC, Guild store, Friend or Guild. They will also get 3 of each mythic along the way to 15 codecs.
This also means, that ZOS would be more liberty to chop and change mythics around as they please. Items that have proven to require delicate balance, should they be nerfed or brought back, it is not a huge issue if the player has 15 codecs on their favorite mythic. Also 0 could be replaced with 5 transmutes if 0 is an issue.