Swapping out the Fusion Drive for discreet drives (SSD & HDD) would help. Keep in mind a Fusion Drive is a dual drive setup as well. In its case you have a small SSD acting as the HDD’s cache drive. Here we would break the fusion drive set, take the small blade SSD out putting a larger one in and then prep the blade SSD as your boot drive also holding your apps but nothing else! You want a drive that large enough so the free space is used for virtual RAM to make up for the lack of RAM. You’ll want something like 500 GB or 1 TB.
But the lack of RAM is the bigger hurdle. This would require a logic board swap-out or finding someone with the skills to upgrade your logic board with new RAM.
Upgrading the SSD won’t be as fast as upgrading the RAM but its the best you can do here without spending a lot.