>>2017 iMac:
Write speed:1872 MB/s
Read speed:2566 MB/s
The Fusion models and SATA SSD ports are no where near that close in sustained real world (AJA, BlackMagic testing) -- the HDD is closer to 90 MBs in its faster cycles -- a Fusion setup in 2017/2019 iMacs is only 3% SSD and 97% HDD -- I would avoid them unless you want to upgrade
I have a 2017 i7 8core 64GB RAM with original Apple FLASH SSD -- it only gets 1831 MBs write and 2085 MBs read Blackmagic
then you have the distinction between SATA SSD (max 500MBs) and Apple 2017 FLASH SSD (max 2000MBs)
the 2017 and 2019 fusion iMacs can both be upgraded to retail NVMe PCIe M.2 SSD with an adapter and sustain 3000MBs speed in the tests
budget wise -- a fusion model 2019 iMac (for the Flash SSD port) -- with the NVMe and RAM upgrades -- would be my choice over the new iMacs unless you spring for extra RAM and larger SSD
the 2017 hardware stops at Ventura -- the 2019 still runs latest Sequoia (both will run Mojave if you need to run older software like CS6 Photoshop)...