Lunar brace + glimmer vs Lunar brace + moonbow

Lunar brace's dmg works the same way as Wo dao, Wrath and Light brand, it deals true dmg which ignores anything, it's not an increase of the dmg you character deals, but just a dmg by himself, Lunar brace+Moonbow isn't the same as Black luna and Glimmer can't increase the dmg.
Moonbow doesn't decrease Lunar brace dmg in any and Lunar brace increased the dmg dealt by AoE specials

You will ALWAYS want moonbow. Glimmer +lunar braze will always be overkill on non-blue units. The math works out so that they are basically interchangeable. It still follows the trend that for lower def high hp units glimmer is better. But due to solar braze, its prety extreme with >20 def and greater than 50 hp. Basically for PVE

And solar braze is calculated independently and then added to the total dmg done. Think of it like how Wa Dao always adds 10 at the end

Lunar brace is actually indifferent to the weapon triangle just as luna and moonbow are indifferent to the weapon triangle.

The thing people tend to miss in the moonbow-glimmer comparison is that it's not about how hard you can hit; it's about how tough of an opponent you can one-round (or potentially one-shot, especially if your name is Lilina). Generally speaking, I find that lunar brace specials hit unnecessarily hard unless you have -atk and don't use hone cavalry (which probably means you're in a squeeze or you played your tactics badly), so brace+glimmer vs brace+moonbow probably doesn't even matter.

As a matter of principle, I think brace+glimmer is probably more consistent than brace+moonbow for the same reason why moonbow+10 (wo dao, for example) has very good consistency in scoring KOs. Moonbow hits hard when the opponent's def is very tough, so when you activate it against some squishy mage, it does almost nothing. Usually that means you're fine anyways because they have no protection against your raw atk, but if you're dependent on the special for extra damage to get through opponents, the +10 can actually help a lot. Now let's look at L.Eirika. If your opponent has poor def, the solution is not lunar brace; it's higher atk (and she already has 56 atk in her base kit without a boon stat, so it shouldn't be an issue). However, let's suppose that her atk stat was something like 45. If your opponent is Lon'qu, he has maybe 17 def and 48HP so the brace is going to give you 8 damage. Moonbow would give 5. The special has no power because it's scaling off his near non-existent def. However, if you go glimmer + lunar brace, he's toast. 45 atk - 17 def is 28 base damage, so 14 more from glimmer, and 8 from the brace. That's a OHKO. So the strength of glimmer + lunar brace is that glimmer hits hard when they have no def and the brace hits hard when they do. You thus get a very consistent result.

Practically speaking, she hits so hard she could be using galeforce, and easily too with hone cavalry, so lunar brace specials I think are best for abyssal maps where enemy HP is stupidly high (in which case definitely use glimmer), healing (aether OHKO, for example), or if you switch out her weapon with the intention of hitting less hard when her special doesn't activate so that they have more HP left for when she activates noontime/sol.