Cable budget in a rig

This is one seriously expensive system. Cost will also depend on the length of cables and power cords, and also on your hearing. In my view, really good interconnects and power cords start at about $1500 and speaker cables at about $2000, digital cable I am not sure, probably $1000 or so. But you should listen if you can, there is no reason to pay more than needed. One brand approach is simpler but not always the best. Some say that if one runs balanced XLR interconnects, especially if the equipment is fully balanced, the cables can be of lesser quality and lower price. Can be in some cases but I don't take it as an axiom. Sometimes $500 cables are overpriced and $1500 are not, and sometimes both are about right, considering overall crazy prices of good stuff.

Yes this will be my end game system for the next 20+ years.

In one system I have, the cables are roughly 50% of the equipment cost to me, not counting any power conditioning.  This is a lower cost office system, and this is where I landed as providing the best overall sound for dollars invested.

In my main system at home, my investment in cabling is currently about 20%, but I would be willing to go up from there if it yielded significant results.

I would say for a system costing what you suggest, you could build a very reasonable cable solution for 15% of the system value, particularly if you are buying some pieces used.  Understand, with the resolution capable in a system at that price point, you may continue to realize significant added value from rather exotic (and expensive) cable designs, and find the comment that cables are just another critical component in your overall design hold particularly true.

Choosing the right cables can be both exhausting and rewarding.

 Good suggestion Elizabeth to try before you buy.