The i5 does still hold up pretty well, but it’s only got six logical cores and no hyper threading. So it’s not too hard to end up core limited, like you’re seeing right now. Especially when you put a 6700 XT on it. Even if you weren’t core limited, you’d likely be bottlenecking it via IPC limitations. You could go for a 8700K, which add hyper threading, but I would suggest getting an i9 9900K for the two extra cores and threads, as well as higher clocks. You might need to flash the BIOS to a newer version to run the 8th gen CPU in your board. If you don’t want to go with the i9, I would suggest going with at least the 9700K, which has eight cores, with no hyper threading. Also, you’ll likely need a cooler upgrade for any of those options.
On some cars the filler neck is a replaceable part… but that doesn’t look to be the case here.