I agree with all the other replies - especially about diminishing returns, hearing how the best headphones sound for comparative purposes, and considering the Focal Clear as another option. I’ll add a couple of further suggestions.
First, if you were to get the Clear, you should consider also getting a different amp than the 789. It doesn’t pair well with the Clear. In this hobby much depends on finding a good synergy between headphones, amps, and DACs.
Second, if you like the 58x, you might invest some of your funds in other amps. You might like to try the Vali 2+, for instance. You might even consider something like the RebelAmp or the Schiit Lyr 3 or a tube amp like the Bottlehead Crack.
Third, and related to the second point, I’m not sure how high the 58x scales. The HD 600 and 650 both scale a great deal with better gear, and you might consider getting one of those if the 58x doesn’t scale much.
In case you’re not familiar with the term, scaling refers to the ability of a transducer to improve with better DACs and, in particular, amps. Some headphones don’t scale very high, which means that you could pair them with totl gear and the headphones won’t sound all that much better than they do with entry-level gear. This stands in contrast to headphones like the HD 600 and 650 that scale incredibly highly such that some folks consider them to be rivals for high-end headphones that cost hundreds if not thousands of dollars more.
Fourth, the HD 600 and 650 won’t do much for you when it comes to sub-bass. They’re great for mids, vocals, and intimacy. Their imaging, to me (and I have both), is acceptable but it’s not a selling point for them - but then again I don’t have totl gear to drive them. The Clear do check all the boxes, though. I’ve not heard it but the LCD-X 2021 might be of interest, too - based on what I’ve seen from people’s impressions on forums. (I’m looking forward to trying the latest LCD-X myself). Finally, and again, be mindful that some headphones and amps don’t pair well together - I don’t know if the THX 789 would work well or nor with the LCD-X 2021.
I hope this helps. Good luck!