In my case, I have two internet connections:
Cellular - fast download, slow upload (Puli/Spitz)
Broadband - slow download, fast upload (currently Slate AX)
Is it possible to download using cellular and upload using broadband to get the best of both? Individually they’re nearly unusable.
I’d also be running a Wireguard VPN.
I have a variety of GL routers I could use, and lots of switches and other APs.
That‘s not possible by design.
Load Balancing does not work like this.
Are there any workarounds or hacky solutions you can think of?
I previously used aggregation/bonding on two ADSL lines in my old home, but that requires identical connection methods (i.e. not a combination of cellular/fiber/copper/etc.), and was wondering if there would be something similar.
Doesn’t have to be load balancing or aggregation.
Nope, can’t think about any real solution for that.
I can’t even think about bonding 2 ADSL lines. Should not work without additional hardware within some datacenter.
I think could be use pfsense which need 3 cables? 
Nah. The problem is that you can’t split connections up between different WAN connections.
You simply can’t send a request via one interface and receive the answer via another one.
You can route different connections to different WAN interfaces, so for example, YouTube will be loaded via WAN1 and Facebook via WAN2 - but this won’t help here.
Yep.
I think the person askign this question, does not have basic knowledge about Network and IP addresses
Its like asking if i can push Gas pedal on my car on a road and want to use the brake on another car for it to stop 