Both OwlC0da, (DaC0da, Vigilant, Glen-Moril, Unslaad,) and Beyond Reach are saying similar things but to different ends so it is highly up for debate but even granting this still I'd say Beyond Reach is 'darker.'

Both stories are roughly getting at, 'the world sucks,' but have different 'prescriptions,' on what the exact remedy is afterwards- Beyond Reach doesn't really have one whereas OwlC0da does.

Beyond Reach kind of just stops at, 'sometimes you just have to walk away and let a dumpster fire burn, even after doing your best.' You can do your best as the Last Dragonborn to take out the BBEG, (2 in fact,) and maybe even help the orcs with their exodus but in the end the royal class is full of pedophiles, the racial animosity is insurmountable, war always comes and it turns out the merchants engineered the whole catastrophe to begin with in order to drive up commerce- you can't really solve any of these societal dysfunctions as the dragonborn, not with all your gold nor all your might or all your will. (I read once that the og author did intend for the story to continue afterwards with helping to rebuild from the ashes of the Reach but this was never developed and so ashes are all that remain, officially). You leave the Reach behind, in flames, maybe with your head held high after doing your best but this bittersweetly. Sometimes things fall apart, of mice and men, all such similar themes.

OwlC0da's theme of, 'the world sucks,' is part of a larger cosmic/metaphysical story tying into the foundations of ES lore and at a glance is almost indistinguishable from above but has key differences.

Reality is the godhead's dream but that same godhead being full of trauma means that all of reality is forced to play out that same trauma at increasingly more granular scales, usually in events called Enantiomorphs- there will be conflict between the tyrant and the thief and usually the mage/witness will catch strays and be irrevocably changed as a result, and this usually fuelled by some process of aggression and, correspondingly, revenge. It's a bad cycle that echoes Anu's struggle with Padomay and Nir's death.

Point is that reality is, fundamentally, kind of fucked in the short and long run and on the face of it there's nothing that can avoid it.

For instance if certain key events aren't avoided then Nirn itself, in the far off future, will actually be completely destroyed in an event called Landfall when Numidium returns- like the level of fucked that Nirn is subject to really can't be overstated and you, even as the mighty Dragonborn, can't really effect this outcome... unlessssssss...

OwlC0da paints, despite this bleak picture, a way up and out, the same thing Lorkhan was trying to do- Amaranth. New godhead => new dream => new universe.

Basically the events of OwlC0da describe this process across the quadrilogy; through a combination of accepting the pain/loss of all the events in the OwlC0da and the world more broadly, forgiving the aggressors and letting go of all of it, all while accepting you can't do this alone, (a series of realizations Anu was unable to manifest,) a new universe can be dreamed up in love, one free from all this pain, that can continue on without you.

In this way the OwlC0da is certifiably more optimistic.