My closest to endgame are Mest and Z1R too, with Monarch MkII for mids
The way you described them is really similar to my impressions although mine is Mest OG, where Mest OG is having slightly more resolution and sharper imaging with the trade of more unnaturally approach, zings and sibilances are more pronounced on Mest OG than Mest MkII, they are more like sidegrades to each other.
And coincidentally my genre also JPOP.
Z1R like you said win on timbre definitely, soundstage absolutely, the detail but non harsh forwarded trebles, where Mest have forwarded mids, and clarity and technicalities.
The Mest imaging is one of the best I ever heard, so special, similar to M9 and u12t or maybe just hairline sharper, all still above Z1R.
About the bass, Z1R have best bass I ever have/try on an IEM, I rated it 9.5/10, where other still nowhere touching 9 score except IE900 and Mest MkII bass, and Mest OG (lower bass quantity than MkII) at 8.5/10. My Monarch MkII bass is on 6/10 but Im very critical about bass quality so my 6 score bass maybe can satisfied others.
The only last 0.5 score bass that prevent me to give full 10/10 mark on Z1R is because of the tuning, a slight mids bass bleed on 100-300hz range although they are tight, so clearly on tuning wise is the last stretch of improvement I want from Z1R bass. on other side, I agree the bass shelf tuning on Mest is better to you and me, but the Impact, the driver quality, the moving air feeling of Z1R is hard to beat, even for Mest.
last month, I tried the Pentaconn Courier Brass eartips, they give more tighter bass, so perceived like less mid bass bleed, with great texture and timbre. And improve the technicalities of Z1R, like having Mest / M9 / u12t imaging on Z1R soundstage imagine how great is that, making a way to my only IEM that almost, very very close to my ideal perfect endgame IEM. the mids also become clearer and little bit forward, but one note the 6khz of lower treble become more open too, made them slightly more intense but in smooth, non harsh presentation (if that an excuse) and also the downside of this eartips is the pricey price tag and earwax magnet, just like xelastec.
They are tacky but not as tacky as xelastec, more like latex H570, although they are little bit easy to pull out from the Z1R shell, but they are sealed greatly.
Yes I have tried a lot of eartips, name it all (my eartips collections are much more than my IEMs) azla any type Crystal, xelastec, sedna max, standard, light, all variants with standard or TWS cariants, spiral dots all variants the OG, the ++ the half one, radius, all spinfits type, dekoni, comply, etc.
For me this Pentaconn Courier beat them all sonically, especially the technicalities. if Spinfit W1, Azla Crystal, Feaulle H570, Moondrop Springs are A rank eartips for me, The Pentaconn Courier is a definite S rank eartips, and the only one making way to the S rank.
Seriously for Z1R user, it is worth a try, especially if compared to hundreds or thousand bucks cables out there, this Pentaconn Courier maybe have more audible improvement over them, and maybe your end game ticket along with your Z1R. Considering selling my Mest and Monarch MkII now.