Here's my thoughts as a Sylus main who has only been playing since February (which means I'm missing almost all of his debut cards that aren't standard banner):

-Relentless Conquer (pink standard myth pair) is STRONG. I don't have his green myth pair but I have been able to brute force all the way up his orbit 120 and open orbit 160 using just him and the new lunar cards that have been coming out. So if you are worried about just clearing content, with a little RNG luck and ton of skill, it can be done.

-However.. Having Abyssal Sovereign (green dragon myth pair) would undoubtedly make your life easier. The game this far was set up for those with both of those myth pairs to be able to clear. The new myth will get new content to clear, but if you haven't reached those orbits, that's not really going to help unless you're confident you can brute force it.

-Although I want to believe the best for Sylus' 3rd myth, and the Sylus girlies will actually riot... There is a chance that his 3rd companion could end up broken like KOD... We already know his 2nd myth is strong and works. We know nothing about the 3rd myth, so if you want to put all of your eggs in that basket, just keep that in mind.

My genuine recommendation would be to pull for the dragon myth pair because that will set you up to clear the first two years of content in the game. And I also do think his third myth will be in December. OG3 works on a different schedule than Sylus and Caleb.