That's a really good question. I've found it's really hard to tap into a senior developer's attention span for any sort of short term work that isn't something they are already working on. To get some non-trivial insight, you're not only asking them for the work you're specifically looking for, but also the context switching necessary to level up on your code base and the work necessary to prepare whatever output you'd be happy with. In some sense, it's like asking someone to live in your home long enough to develop their own thoughts on how they like your home, and what changes they might make. It's hard problem to solve.
Another approach to consider is simply not worrying about it too much, only worrying about making serious quality improvements when the project starts to get traction. I once asked a senior engineer for their thoughts on how to test a very specific type of coding approach, and their response was that it didn't really matter until the code was important enough that it needed to be taken seriously.
For me, I'm banging on an Elixir app this weekend. I'm intentionally moving slowly on it because I'm working on refining my approach to how I structure and test the code. In many ways, I find going super deep on a fairly small thing makes it easier for me to find patterns that I'd like to replicate else where. So I'd say my approach to finding code review is to simply budget time to become my own reviewer. Another benefit of this approach is that I often find I develop thoughts that are efficient to communicate, and then I'm able to share them with coworkers as the opportunity arises. My better ideas tend to start getting adopted by others, and then they get better at reviewing my code.
For Elixir, I've also found people who are interested in pairing on the Elixir Slack channel. [1] Consider looking around with your language or framework of choice to see if there's a community where you can find people interested in pairing with you.
[1] https://elixir-lang.slack.com
