dual boot with two SSDs: ubuntu + ventura on old mbp

Hey there! I got a question ChatGPT couldn't provide decent answer ;-)

I was using Ventura with OCLP on my old 15" mbp (late 2011, reflashed gpu mixer, so no dgpu - if it is important). Once I needed linux, I removed the old SSD and installed a completely new one and then installed Linux Mint on it.
Recently I purchased an optibay and installed an SSD with OCLP Ventura instead of superdrive. The problem is I could only see the disk picker (booting with 'option' key) once - now with option key I am getting just the chime and the black screen. WIthout key pressed I can boot into Linux Mint. I am not that experienced in system administration, but I am sure there should be a way to choose the OS on startup? Can anyone provided a detailed instruction?

Yet another question is I though that linux community had a simple way to browse APFS volumes, but it seems I was mistaken - I couldn't make it work on my linux mint ;-(