My external drive connectors don't fit in my new iMac

The Seagate SRD00F1 comes with a proprietary cable with a USB-A end. You need an adapter like this one to connect it to your Mac -> USB-C to USB Adapter - Apple or like this one from Monoprice.

Toshiba, Nexstar & Green hub - can't tell, not enough info; would need the exact model #'s. But I am going to guess that they also have cables with USB-A ends, so you would need the same adapter for them.

Here is a pictorial guide to the various USB connectors -https://www.cablestogo.com/learning/connector-guides/usb It may help you understand what the different types of connectors look like.

You need a USB-C to USB-A adapter to go directly to the iMac.

of the genuine Apple adapter

USB-C to USB Adapter - Apple

for some aftermarket one that will also work with the new Mac's

https://www.amazon.com/dp/B07PDLQ555/ref=sspa_dk_detail_0?

https://www.amazon.com/dp/B08Z768LCW/ref=sspa_dk_detail_1?

https://www.target.com/p/philips-usb-c-to-3-0-usb-a-female-adapter/-/A-79804275

Please tell us exactly what model iMac you have and the exact model external drive(s) you are using. Also the hub.

Without that info no one can tell you what cable(s) or connector(s) you need.

To get the model of your iMac, do Apple > About This Mac. Tell us what it says. Here is an example:

Talk to OWC (MacSales.com) Customer Support to see what they recommend in the way of self powered hubs for your particular model, workflow and budget. The have numerous models and are considered the premier 3rd party hardware supplier for Macs by most of the experienced users here.

The slowness could be from the individual drives and/or the hub (check all their specs). Look for what version of USB each supports:

USB 3.0 - 5Gbps

USB 3.1 Gen 1 - 5 Gbps

USB 3.1 Gen 2 - 10 Gbps

USB 3.2 Gen 1 - 5 Gbps

USB 3.2 Gen 1x2 - 10 Gbps

USB 3.2 Gen 2 - 10 Gbps

As for the Green USB C Hub - if it's rated at one of the 5 Gbps versions of USB then everything connected to it will be limited to 5 Gbps max speed even if the connected devices are faster.