I've had that exact case before

it's built like a brick $hithouse lol. Forgive the term if it doesn't exist in your area, basically it means 'built like a tank'.
The ventilation is pretty bad on that guy at present. Do you have access to a Dremel and appropriate attachments for cutting thin steel and aluminum? The front of the case is stupid thick, so don't worry about that. Perhaps on the side panel you can cut a 120mm circle and drill four screw points to account for a side intake somewhere low to clear the GPU. The rear exhaust iirc on that thing is 60mm and breathing through a very restrictive pattern of holes. And of course no top ventilation.
Cutting the rear 60 to an 80 and knocking out the metal that's in the way will dramatically improve airflow, if combined with a new side fan and new 80mm fan. You can get little grills and foam inserts to block dust to go with the fans. The final step would be to see if an 80mm fan would clear in the top center of the case for a top exhaust. 120 is probably too big, but 120in + 80out + 80out should make for a pretty adequate non-OC airflow situation. Basically the biggest things that have changed since that case was 'current' is that GPU and VRM capacities have gone way up in terms of surface ICs that need to be kept cool. That GX5050 case is from 2004, and was designed for 100W or less GPU, and sub-100W CPU/Mobo. 2004 was the year of the Nvidia 6 series. Even the flagship 6800etc models were less bulky and hot as your typical RTX2060, a mid-range card. And the Vega is definitely something to keep an eye out for in regards to temperature.
Perhaps leave the side panel off for the time being and point a little fan towards it. Stagnant air sitting around on the Mobo VRMs and GPU PCB are what would concern me about a case with functionally zero airflow as it presently exists.
And if doing all the cutting and modding doesn't sound like something you want to mess with, perhaps just keep an eye out for a newer case. Yours is 16 years old. You don't need glass panels and all the LED nonsense that is everywhere with new stuff, but cases from 4-6 years ago will be far far far better for what you've got as a build there. Ideally something that has USB 3.0 front panel header, enough intake/exhaust fans and/or mounts, and a bottom side PSU layout (this opens the top of the case up heat to rise and exit instead of a big chonky PSU blocking that possibility). Bonus points if there is a decent backside access to route cabling under rhe Mobo area, which makes keeping the tangle issue much less troublesome.
I admire the commitment to making something work, but I do think it is probably time to call that case done unless you want to do the mods to raise it's potential.