MacBook Pro 2020 with 2 x Dell U2518D monitors
Your issue is very much like the one in this question about a MacBook with only two USB-C ports as well:  MacBook 4K/60Hz adapter is working directly, but not via USB-C hub
While you have two Thunderbolt ports, the MacBook only had USB ports that supported DisplayPort.  So, in your case, you have a couple of options:

Plug one display into one of the USB-C ports and use the USB-C Hub w/ Power Delivery as described in the answer in the above linked question to connect your devices and your charger.

Use a proper Thunderbolt 3 Dock.  The problem that you're having with the Belkin Thunderbolt 2 dock is that while Thunderbolt is technically backward compatible, Thunderbolt 3 uses passive cables where as Thunderbolt 2 cables were active.  A number of &quot;high bandwidth&quot; devices like storage and display require an active cable which the standard USB-C cables are not.
I recently wrote an answer about this for the question LG UltraFine &amp; replacement Belkin USB-C cable gives blurry text.  Basically all (quality) USB-C cables are Thunderbolt 3 cables.  However, not all are active.  Active Thunderbolt 3 cables carry power and cost about $50USD more than regular ones.  I believe this is why your Belkin dock isn't working properly.


I would go with the first option: using the USB-C hub with PD.  This gives you a lot of options out of the gate without spending too much money.  If/when your needs outgrow the hub, go for the Thunderbolt 3 dock as you'll have much more room for growth.
You can try getting the active TB3 cable, but to me, it's more of a bodge and I'm not convinced the Apple TB3 to TB2 adapter carries power.  There are lots of folks looking for Thunderbolt 2 docks because they have 2017 and older MacBook Pro laptops they'd like to expand but can't because of product availability; it could fetch a nice price online and help pay for that Thunderbolt 3 dock.