P550 cores don't have vector extensions. It's actually quite an old design, from 2021. What you'd want is SiFive P670 cores, which are RVA22 compliant with the vector 1.0 spec.
Arm A53 (Pi 3, October 2012 - February 2016), A72 (Pi 4, Feb 2015 - June 2019), and A76 (Pi 5, May 2018 - September 2023, or January 2022 for Radxa Rock 5B) all took longer.
P670 was only announced in November 2022. If a board ships by the end of 2025 it will be doing very well.
Traditionally in x86 Intel and AMD do all the first three steps in one company, with the stages overlapped, and feedback.
Also, Intel and AMD (and even more so Apple) don't announce a new chip until it is very close to shipping. They might have been working on it internally for five years before that.
Arm and RISC-V companies have to make public announcements when a core is nearing design completion, to give a chance for companies such as Allwinner and Rockchip and Broadcom and Mediatek and Sophgo and Starfive to take a look at the specs and decide that it might be interesting to build a chip using that core.
> I know the round trip time from the foundries is ~3 months.
That's only if the chip works first time. Many don't and need several re-spins. I believe 3 or 4 is not uncommon. And variable amounts of re-design and re-layout and re-verify time between each of those ~3 months at the foundry.
Given that, it would be a brave company that went straight to mass-production without a round of test chips first, so you've got 2x ~3 months, plus "bring up" time in between, even in the best case.