The Pixel 3 does not have a slot for inserting an Micro-SD card. Therefore you can not upgrade your device.
The 64GB memory chip is built-in soldered to the main board of the phone. Replacing it is next to impossible.
The only workaround would be using an USB flash memory stick with USB-C connectors (or an USB-A version + adapter cable) and manually copy ...
Issue you're facing is mostly because of either of these two reasons:
MTP acts retarded when there are too many files
Windows acts dumb sometimes (switch to linux)
I don't really have a solution to either than switching to linux (as i did) for the latter
Now some REAL workaround to HELP you:
Connect Your phone and computer to same WiFi/hotspot or even ...