It would be consistent with previous Relics though.
Where the lore was always "We need this really rare item" then it happens that Rowena, the person that has everything for sale, has the item and is willing to sell it.
It also makes things fair. If you had to purchase with Tomestones, every item, then you'd be limited to your weekly cap irregardless of which type of content you do. Which means everyone is gated the same way and as such, won't feel "Pressured" into doing the "Most efficient" content.
Which is one of the main issues with more freedom of content, is that people don't like feeling pressured into grinding the most efficient stuff, which is why there are so many complaints about 2.0/3.0 light farms because people "Spammed Trials/Tam-Tara" as it was the most efficient way of farming the light in a short amount of time.
Again, tomestone purchases are consistent. Rowena has items, you purchase items, you give items to Gerolt (Who's in debt with Rowena), Gerolt makes you a fancy weapon (To pay off his debt).
This has been consistent since HW (In ARR he wasn't in debt with Rowena, at least not heavily enough to be coerced into working on your weapon. He just wanted to make his kettles to pay her back)
Note that most of my suggestion actually had a single path. One in which you buy items with Tomestones.
How you earn those Tomestones are up to you.
As far as my side comment about the stats, well, it's no different to how ARR required you to gather up a bunch of Materia to put in. How you got those materia and which materia you used was up to you. Similar thing for Crystal Sands in HW where you had a plethora of items that could be traded in for the sand, from Scrips, to Leve loot, to raid items or stuff bought with poetics or even GC seals.
As such, it's not unprecedented for multiple ways to progress a single step. It's only this Eureka relic that has a singular path. Where the ONLY way to progress it is by farming Fates or Farming Light/Logos in the relevant zones.
So, something simple, like the items you use to put the stats on can also include content specific stats such as Elemental Bonus in Eureka or extra Equipment levels in HoH is not out of the question. As it would simply be a similar thing to how ARR's materia step worked, where you just need to apply a certain amount of X, Y or Z where X is found in Dungeons/Trials/Raids, Y is found in Eureka and Z is found in HoH and where Y grants additional Elemental Bonus in Eureka and Z grants an additional bonus in HoH.
Again, it's not hard to think of things like this, because, as it happens, both relics before SB literally worked this way, wherein you COULD progress a majority of their steps in different ways.
People like to complain about being "Shoehorned into farming old content" from ARR/HW relics but as it happened, specified duties actually made up a minority of their grinds. The majority of their grind was about gathering Tomestones and trading them to Rowena.
Even some of the non-specified duties could be bypassed by just instead spending Tomestones (Such as HW's Singing Clusters or ARR's Alexandrite which either required a daily level 50/60 Roulette or purchase via Tomestones)
How you got these tomestones, the game didn't care. Where you went to farm your Light, the game didn't care.
The fact that an argument against allowing progression on the Relic from outside of "Eureka" content is "They won't do multiple paths to the same item" just boggles the mind when literally, before Eureka, that was exactly the case, outside of like 2-3 steps of each Relic.