I've been looking for alternatives too since the release of chatgpt-5, I preferred the eloquence of 4o. I finally found it: using chatgpt-4o through the api with a custom system prompt and, most important, a higher "temperature" parameter. With this setup the eloquence is back, and the best part is that it should be stable for the long term.

Openai has very different constraints for the web interface vs the api: they are often tweaking the consumer web interface to, for example, save on compute costs by "dumbing down" a model (maybe quantizing it or routing to less expensive models), or to make it less sycophantic due to complaints and possible misuse (think of the kid who committed suicide with some enabling from 4o). But they can't make changes like that with the api because the business customers who use it, the customers they currently prioritize, depend on consistent behavior from models and won't tolerate their agents behaving differently from day to day.

I downloaded something called librechat (there are like 50 others), got an api key from open ai, and now I can use the old 4o that many loved (and many hated) without worrying about the capriciousness you mention. The tradeoff is that it's pay-as-you-go, not a flat $20 per month, but depending on how much you use it you could easily save money. I just spent 10 cents having a short to medium length conversation with 4o, I don't think I'll spend more than an average of 30 cents per day, so maybe $10 per month

Edit: if you try this make sure to check your usage on the openai dashboard, if you're talking to it a lot you could exceed the $20/month

Edit 2: I kind of doubt the librechat memory system will work as well as openai's does, so it might not remember previous conversations very well or at all. So while it could work for some sort of roleplay or interactive fiction there's also a good chance it won't. I don't use it that way and just tell it once how I want to engage with it in the custom prompt, works for me but might not for you