Drew, I'm using only a 2017 5k iMac (Fusion storage) and find that having only 2 Thunderbolt 3 ports is quite a challenge when it comes to using more than a couple of external SSDs. The iMac Pro had 4 TB3 ports but it's not currently on offer in Apple's lineup. The latest 21.5" iMac (with Apple Silicon) has only 2 TB3 ports.
Looking ahead to when my iMac's internal storage has expired, I have a plan. I'll use one of my TB3 ports for a LaCie 1 Big Dock with built-in conventional fast hard drive (for bootable clone backup), daisychained out to a 2TB Samsung X5 for the system plus miscellaneous files. On the iMac's other TB3 port I'll put another 2TB external SSD for sample libraries. It's not ideal and I could well run out of SSD storage space before long - then I'll be up the creek without a paddle, lol.
There are so-called Thunderbolt-SATA caddies out there but mostly they're just for those higher USB speeds which are only available via iMac's TB3 ports. A lot depends on the speed of your SSDs and the version of their SATA interface: if they don't need more than USB 3.0 speed then the choice of USB-SATA caddies is wider and you might find exactly what you need without having to use a precious TB3 port.
Moreover, although TB has the potentially very useful feature of being daisychainable, in practice I don't think I've ever seen any external SSD with provision for this feature - the extremely expensive LaCie 1 Big Dock SSD Pro being a notable exception (but, as the name says, it's an SSD plus TB3 dock in one enclosure).
Decent TB3 docks without built-in hard drive or SSD are expensive and not abundant, and I don't seem to recall if any had true TB3 daisychaining (sorry, my memory might be at fault there, and I've not been searching in recent months so things might have changed). The worst problem it seems to me is, until external SSDs add a TB3 thru-port we'd need dock after dock, each with at least 3 TB3 ports (1 in, 2 thru), to be able to connect to multiple 'spurs' in the daisychain, and thus far I know of no dock capable of this.
Short answer is, sorry to say, it's complicated. I'd suggest you prepare yourself with a bit of research on all the various USB and SATA versions, then it's just a case of slogging through Amazon etc to see what's out there today. Also, I'd urge you to be very careful about exactly which Thunderbolt version a device on offer is designed for. TB1 (officially called just "Thunderbolt") and TB2 are compatible with each other but not with TB3, although a rather expensive active adapter is available from Apple for using TB1 and TB2 devices with your new iMac.