Bear with on this as I don't have the same gears as you have but I were in your shoes a few weeks back. I'm lucky enough to audition both units to my heart's content. I used an EE ESR MK2 to audition both. I don't own a Monarch nor have I auditioned it.
Both units will do DAC/Amp functions for your PC/Android phone. If you're huge on Tidal and you want full MQA decoder then +1 with the Gryphon. MQA on the Mojo 2? Mojo 2 answers - what's that?
If you want to play around with EQ, +1 for the Mojo 2 as it has full EQ function (although you might be confused with the instructions). EQ on the Gryphon is practically non-existent - does this bother me - no since I don't play around with EQ.
My ESR MK2 hisses by just a tad on my Gryphon, does that bother me - no. Because I can't hear the hiss when I'm already playing music, So I never enable IEMatch - athough it's good that I have it now should my preferrence change in the future.
I used to own a Mojo (1st) and a DX50. The Mojo does have a "Line-in" function - you just use the Coaxial input which the DX90 has - a Coax out.
I just got back to the hobby after 3-4 years of hiatus. So my previous DAC/Amp left was a Centrance Mini-M8.
What did I purchase - the Gryphon. Why, because of it's versatility, how good it pairs to my new gears and great aftersales feedback. It has Balanced which majorty of my gears have balanced plugs. It has Bluetooth, if I feel like going wireless, I have that capability. Wireless on a Mojo 2 - then you still need to purchase the expensive Poly.
Hope this helps your decision making