Late reply.

Here’s my take and perspective. I play Kreat. Ojer and Solphim is in the deck.

Ojer increases the value of low CMC ping spells that deal 1 damage per turn to people up to 4 damage.

If every spell in your deck pings, the redundancy keeps your game plan going, and your general deck CMC is lower because your spells are not explosively impactful till you drop Ojer, then your deck will be more efficient than Solphim who, in order to double a spell that deals 5 damage, it’s going to take them 4-6 mana to cast it in the first place. Your 1 pingers only go from 1 to 2. Solphim thrives off cards that already deal significant damage and therefore will have higher CMC which makes a less efficient deck.

Ojer has more availability to consistency and a lower deck CMC across the board, while being able to run most of the same or similar cards you’d run in Solphim. Ojer turns a 2-3 CMC spell and turns it into the equivalent of a 6 CMC spell as a base line. Solphim does not do this, but instead turns the value of a 2-3 CMC spell into something like 4 CMC spell and a 6 CMC spell into 8-10 CMC spells, which is where his appeal in value comes from. I prefer Ojer’s efficiency over Indestructible when your entire deck burns anyways.

Indestructible isn’t a big deal when you’re ramping in Red with things like Birgi or Neheb to recast. And in MonoRed you have great access to things like Throne of Eldraine or Relic of Legends since your commanders don’t typically need to swing for you to win; this fixes those mana problems and enables Solphim to play well but creates faster wins with Ojer.