Google AI Plus is not the equivalent to most other paid plans, it is closer to ChatGPT Go. 3.6 flash is an equivalent model to luna 5.6, which is the highest available on ChatGPT's free & Go plans.

3.8 flash has been perfectly available to Pro users from the announcement day.

You are on the cheapest paid plan and complaining that you don't have access to more expensive models. (Though idk why you only have the lite version, I have access to the non-lite version even on the free gemini plan as long as I'm logged in.)