How would you rule a potion of flying? Does the imbiber have to concentrate?
I would say the ring of spell storing uses the original casters DC and level. but it should rather work as a potion than a scroll read by the actual players char. So no concentration imho and no metamagic. Just replace the wording cast with unleash the charge. I am sure that this is for some reason not RAW but I would rule it like that.
Because for me as a DM a ring of spell storing is not an alternative to a scroll of bless only readable by the party cleric but an item to give a character a (or some) one shot spells he has no access to otherwise, no matter what class the PC is.
If I want to give out additional fireballs to a wizard I will let him find a wand not a ring of spellstoring. A ring of spell storing makes more sense from a plot view as a dm to allow e.g. for one or two single teleport spells or a disintegrate spell intended for an item or a via wall of force or a plane shift or a word of recall you get what I mean.
Regarding concentration:
According the Dungeon Master's Guide:
Some magic items allow the user to cast a spell from the item. The spell is cast at the lowest possible spell level, doesn't expend any of the user's spell slots, and requires no components, unless the item's description says otherwise. The spell uses its normal casting time, range, and duration, and the user of the item must concentrate if the spell requires concentration. Many items, such as potions, bypass the casting of a spell and confer the spell's effects, with their usual duration. Certain items make exceptions to these rules, changing the casting time, duration, or other parts of a spell. A magic item, such as certain staffs, may require you to use your own spellcasting ability when you cast a spell from the item. If you have more than one spellcasting ability, you choose which one to use with
the item. If you don't have a spellcasting ability-perhaps you're a rogue with the Use Magic Device feature- your spellcasting ability modifier is +0 for the item, and your proficiency bonus does apply.
DMG 141 (emphasis mine).
In the July 7th episode of Dragon Talk, Jeremy Crawford discusses magic items. Starting at 21:37 he discusses the reasoning behind the rule that a spell cast from a magic item that requires concentration requires the user who cast the spell from the magic item to use his or her concentration.
"If it is a spell that requires concentration, does the user of the magic item have to concentrate when they cast the spell from that magic item."
"The answer to that question is you do have to concentrate, unless the item says otherwise. And there are a few items that say you don't have to concentrate."
He says that they this is an issue they predicted when working on the DMG, which is why it is in the book.
"That is something that is at the heart of our design. Not only spell casting, but also with magic items, because you'll see how we limit concentration spells, how atunement works with magic items, we also want magic users in our game to have to make interesting and sometimes tough choices."
Potions are not an exception unless the descriptions specifies. For example:
POTION OF GASEOUS FORM
Potion, rare
When you drink this potion, you gain the effect of the gaseous form spell for 1 hour (no concentration required) or until you end the effect as a bonus action. This potion's container seems to hold fog that moves and pours like water.
With a potion of flying, however, the rule doesn't state that you gain the effect of the fly spell. It states that you gain a flying speed equal to your walking speed for one hour. I would argue that you would NOT have to concentrate.
Actually, it seems if most if not all of the potions in the DMG that have the imbiber gain the effects of a spell of concentration state that no concentration is required. None of the potion descriptions describe the imbiber as casting a spell, merely gaining the effects of the spell. To keep consistent with the general rule, they specifically state that no concentration is required when gaining the effects of a spell that would require concentration if cast.
This seems to be major benefit of potions over other magic items.
A ring of spell storing, however, seems to be clearly be an example where concentration is required, based on RAW.
A DM, however, is free to create magic items that operate differently.