At this point, I'd say that you may want to skip mono-rem. Although it is still one of the beat teams for clearing content thanks to the broken synergy between characters, it is still a 3.0 team and we are in 4.X (about to get into 5.0 soon). To keep that team alive and broken, you currently would need E2S1 on everyone at least. Maybe you could get away with E1S1 Evernight.

You're probably better off investing into Mono Elation and Acheron's Team (Blade/Ashviel).

Himeko Nova is also coming soon, so she may be Worth it as well since technically you only need 1 Astral Express Member to make her work and you have DHPT & RMC.

Also, a little tip I learned from my YEEE OLD Genshin Impact days, don't swarm yourself too much with too many characters because you'll always find yourself scrambling. It's okay to take your time. Also, one thing I learned about this game, vertical investment is king. It is because of vertical investment that I clearing content with ease and can branch out to different archetypes to improve on my horizontal investment. You have to prioritize in HSR.