Okay, here’s my blog post about figuring out Sol3 Phase 5’s level, written in a casual, personal-experience style:

Alright, so I’ve been banging my head against the wall with Sol3 lately, specifically trying to figure out what level that darn Phase 5 is supposed to be. You know how it is – you get into a game, start making some progress, and then BAM! You hit a wall. That was me with Phase 5.
Digging In
First, I started by just playing. I mean, that’s the obvious thing, right? I figured if I just kept at it, I’d stumble upon the level requirement eventually. I played through the earlier phases a bunch of times, made sure I was collecting everything I could, and generally just tried to max out my stats. No dice. Phase 5 was still locked tight.
Then, I thought, “Okay, maybe there’s something I’m missing.” So I went back to the main menu, poked around in the options, and checked the in-game help. Nothing. Seriously, nothing at all about level requirements. Talk about frustrating!
Getting Help
Next step? The internet, of course! I started by just straight-up googling “Sol3 Phase 5 level”. I know. real creative. Most of it was about all the things, so It took me about 5 minutes to find someone saying that my level is too low to play Phase 5, my level is at 12 that time.
So I kept searching for the answer, after scrolling for like 10 minutes, finally, I saw the anwser!
The solution!
Finally, I found out, after I had tried many times, I realized I need to reach level 15, oh my god, so that means I still need to grind 3 more level.
Eventually,I did it, I reached level 15! and yeah, I can play sol3 phase 5 right now, what a long journey.