As someone with a tumultuous relationship with Google, I can attest to what has worked for me as far as creating an anonymous as possible Google account and troubles I have ran into when attempting to do so. As suggested above, it is best to attempt to create a new Google account with a fresh Play Store install on your device. Having Randomized MAC Address - Per Connection enabled on your GrapheneOS install settings may also help, although I'm not entirely sure if it has any actual impact on creating a new Google account. When creating the account, I used a username as my first name, omitted a last name, and when given the option of using my own address or signing up with a new Gmail address, I chose gmail and opted for one of the automatically created addresses Google made based on the first name data I provided. That went without a hitch as I was able to bypass the phone number requirement. I used a freshly created Protonmail email address as my recovery email address (no PII in the email address). No VPN was necessary, I did this all from my home network.
YMMV with the above method, as I ran into issues creating new accounts before even though the phone number requirement was bypassed. Whether or not Google will require you to provide a phone number upon account creation seems to be very much 50/50. I was met with CAPTCHAs in one instance and had an account suspended for no reason whatsoever in another, I wish I could say for certain what caused these issues. Perhaps Google keeps count of how many new Google accounts are created from a certain IP address? That's just me speculating, admittedly, I have no evidence this is at all the case.
In other instances, I had no issues creating accounts using a combination of JMP.chat numbers, falsified PII (pseudonym, fake DOB), and keeping my device connected to a Mullvad VPN server. To elaborate, one account was made while connected to a Canadian VPN server, with a Canadian JMP.chat phone number, and falsified PII. Another was made while connected to a US city VPN server, JMP.chat phone number with area code from that same city, and falsified PII. Again, I can't stress how much that there is no guarantee these methods will work. Sometimes it really does seem to come down to luck of the draw when creating a Google Account. As to why I created so many, I go back and forth between wanting to completely kick Big Tech out of life, to realizing I have to rely on it to some extent for practicality's sake. Hope some of this helped someone in some way!