Croatian citizens have been eligible for ESTA since December 1, 2021have been eligible for ESTA since December 1, 2021, so even if you did not have a visa, you could use your new Croatian passport to apply for ESTA.
Instead, you should travel with both passports: the valid one and the expired one that contains the valid visa. As to veracity, here is what the US government itself has to say about it (from the FAQ page About Visas - The Basics):
Do I need to apply for ESTA if...?
I have a current, valid visa?
Individuals who possess a valid visa will still be able to travel to the United States on that visa for the purpose for which it was issued. Individuals traveling on valid visas are not required to apply for an ESTA authorization.
You also ask:
I just read that some sites don't actually "scam", but just charge extra, but still result in you receiving a valid ESTA approval. If that is the case, would it conflict with the ESTA application I submitted myself, afterwards? Is there a consequence to applying twice?
It's perfectly fine to have a valid ESTA and a valid visa. In your case this would mean that you can choose to travel with the visa by using your Croatian passports or with the ESTA using your German passport.
As Hilmar notes in a comment, application for a subsequent ESTA should result in cancellation of any prior ESTA authorization for the same passport. But I would recommend, if you have submitted more than one, to double check before traveling that at least one of them is still valid. You shouldn't really need to keep track of which one it is.
As Hilmar also notes, a traveler with a visa has more rights than a visa waiver traveler. The only rights that are likely to matter are the period of admission (a default of six months vs. an absolute limit of 90 days) and being able to apply to extend or change your nonimmigrant status without having to leave the country. These rights matter to very few tourists.
(The most prominent right that matters to almost nobody is the right to demand a hearing before an immigration judge if you are refused entry or identified for removal -- i.e., deportation. Most tourists won't be in either category, and most of the few who are will be unlikely to want to challenge the decision before an immigration judge.)
