This is normal. N-400 cannot be decided without I-751. Inquiry is not pressuring anybody. The only thing that would force things to happen is a lawsuit, Writ of Mandamus. But typically you won't sue before N-400 interview, unless you want to sue for I-751 decision. For which 21 months is not that surprising. Mine took about this long when thing were faster.
To give a perspective, I filed N-400 in December 2024, I-751 was approved in 2023. I'm not expecting decision until November 2025 or early 2026. One year is reasonable for N-400 without pending I-751. Elections are over, there's new administration coming. USCIS priorities are likely going to shift from naturalizing peope fast to other things.'
If I filed in March 2024, I'd start worrying after May-June 2025.