You need to tell us budget you are ready to pay.
Best possible custom loop for CPU and GPU with very limited hardware I measured cost 600$ for CPU and GPU.
But only straight fittings, balbe valve, best radiator, CPU and GPU blocks, D5 Pump Combo, etc...
If you buy ASUS products they have good partnership with EKWB, and together work great.
I will first suggest you to install CPU watercooling with space to add GPU later.
For me 13/10mm tubing is best 3/8" 1/2".
I even thought to buy 10 Bitspower fittings from Caseking.de for 44$ but 11/8 size, they look perfect.
But tubing options stopped me. No Mayhems Ultra Clear, no EK Dura Clear, nothing proven.
Caselabs not work any more. But you can find Lian Li. Prices in USA much better than in Europe.
I will never bought other PC case, except Lian Li, and only aluminium. I cursed in house when I saw Lian Li PC-O9 WX for 314$.
I had arround 300 euro max for case, but he costed 450 euro in Europe. Now sit on Amazon and wait someone with taste to pick him.
When you deal once with Lian Li aluminium other cases are not worth any more, no matter how they look. Caselabs is better, but they abandon market and that's one of biggest loss I remember in IT industry. Many people dream that one day will have 500-600 euro to pay CS, I liked Gunmetal S8S Limited, smaller variant of S8.
Radiator depend of cases. Single 360mm is minimum for CPU and GPU.
2x 360mm is best scenario for single CPU and GPU.
Depend of size I only could recommend HW Lab Nemesis GTS or Nemesis GTR.
Coostream PE look nicest on market but core is much smaller than you see, very similar to thinner radiators and for their size perform great.
Flaw is thin aluminium housing. Nemesis from other size look like you can smash wall with him or other radiator on two peaces.
GTR is some new 4th Gen design who work insanely good with my fans type and I recommend them to you Noctua NF-A12X25. I bought 3 for 70, guy didn't want to sell 6 for 150. Best fan shooping I ever done. They are so good that color become interesting.
Buy CPU and GPU block EKWB, Reservoir and EK Pump D5 G2 PWM or D5 Lowara.
radiator Nemesis GTX or GTR if you want thick, GTS if you want thin... GTR work with faster fans 1500RPM+ and perform best, other with slower fans.
Fittings depend of taste, reliable are EK, Koolance, Bitspower, Swiftech, Moonson, that really depend of taste.