the best first split keyboard

It really depends on what you're most comfortable with; when you go for such a custom option most of the design decisions are about personal preferences.

I suggest you draw out some layouts on a piece of paper, adjust them until you feel happy and then plan out how you want the keymap to look. When you're happy, look for a layout that fits what you want or build your own on KiCAD.

I bought a kyria from Splitkb, and I've been very happy with the design. If I needed another keyboard, it would probably be a very similar layout, but have slightly fewer keys, be low-profile and no oleds.

Lily58 or Solfe. If you want the Corne experience, just remove switches.

Consider something that allows you to do both Choc and MX unless you already have a strong preference. LilyPro can do this.

The extra column also allows for the pinky mod, which is my favorite Miryoku layout (moves the top pinky column outward). Pikachoc has this natively.

I definitely vote for the Piantor by Beekeeb

I chose to build a Cantor (https://github.com/diepala/cantor) partially because I liked the diodeless design, the thumb cluster that is a bit splayed out and I didn't want to commit to a 3x5 layout like in the cheapino. I pretty happy with it.

My first split was the sofle choc v1.2. I maybe wouldn't recommend it over the v3 now that I've tried both but both are still very good!

I liked the SplitKB Kyria. But my wrists did not (through no fault of the keyboard - wrist injury).

Split keyboards are niche enough that the cheapest option I found was building it myself (with pcbs by jclpcb). Ones I looked at are corne, cheapino, lily58, and ferris sweep but I settled on the chocofi and have been loving it

Edit: my goal was to find something "corne-like" as there seems to be good resources for it and I wanted to try the miryoku firmware

I started with an Afternoon Breeze (https://www.afternoonlabs.com/breeze/). I have since built a Sofle RGB (https://josefadamcik.github.io/SofleKeyboard/build_guide_rgb.html), with a second Sofle RGB planned to leave at the office.