There's no sign more complicated cases get left until last.
The time you can apply for an inquiry has always been fluid. They delay it if they get too many as it slows them down. It's not always after the processing time is passed.
I understand it may give you comfort to be acknowledged by USCIS that you're still waiting but they know that, you're in the system as pending and they'll get to it when they do.
Remember responding to inquiries takes time so if they're having to delay when you can apply that means they're being swamped by it. They're not going to message someone saying 'hey they're still waiting btw', it does nothing but give you comfort.
I've never seen a post from anyone being forgotten by uscis so I don't think it's a real risk and you need to remind them you're there, if you're in the system, they know. The case numbers reflect when you applied so they can tell straight away now long someone is outstanding.