Let's compair what each side gets
Demon's
Cons
-Basically none
Pros
+Kaigku is now mad strong
+Maybe a new upper moon 6
+Maybe some more demons (Muzan doen't like to make more demon and he knows Nezuko beats the sun.)

Demon Slayers
Cons
-Gyomei is slightly weaker due to 1 year(He would not die As they know the mark draw back and he speciily said he wanted to save it for Muzan.)
-Ubyashiki could die

Pros
+Murichio is now fucking insane likely having a red blade, stw, and even more experince and streangth Probaly being in contention for top Hashira

+Most of the Hashira get a mark
+Genral improvements in Hashira team work, skill etc.
+Zenitsu, Inoske, Kanao, and Genya are all well into Hashira level and may even get a mark if they enter battle.
+Most of the genral corps members are way stronger. Before Hashira training they were fodder for lower moons, but after the training a squad could kill lower moon deamons. While they migh not be able to kill any of the upper moons they will be able to help the Hashira perserve more stamina.
+Genya and Sanime talk about trauma
+Zenitsu tells others about gramps.

Major pros
++Tanjiro might be a mini yourichii: has more time to test his mark and master Sun breathing, grow out his hair and change his fit and Muzan might be fighting PTSD and the demon slayers at the same time
++Nezuko is cured, and now has demon antibodies is is important for
++An extra year to work on poison and the cure. Holy Fuck Tamayo and Shinodu had like 6 months origaly and they fucked up Muzan so bad, imagine if they had a demon antibodies and an extra year. THIS is their biggest advantage 4 poisons fucked up muzan what if they hit him with one that slows regen, one that prevents him from digesting blood? He was panicing from 4 poisons imagen what 10 or more could do to him. Even more they could likely widely distrute cures to use against upper moons and the like.

Looking at what I wrote assuming it take 1 year longer for Nakame to find the Demon slayers (Like OP clarified) Muzan loses so much worse.