Are you looking for a complete custom water cooling loop or an AIO solution?
For a Ryzen R9 3900X i would prefere a AIO version with 280 mm radiator or higher. But a 280 is enough. Eisbaer or Eisbaer Aurora with 280 mm or Eisbaer LT with a 360 mm radiator.
Makes no big different wich AIO you choose.
True.... easiest way So choose what you want. Like i said.... take a 280 or 360 mm radiator version. Wich version depends on wich design you like more. the Eisbaer Aurora is a bit better for Ryzen CPUs but the differences are minimal.
The main difference between Eisbaer and Eisbaer LT is the missing reservoir and the LT version has 25 mm thick radiators, the normal Eisbaer has 30 mm radiators. But the 360 mm LT version is strong enough for all CPUs on the market and the reservoir is mainly interesting if you like to built a bigger loop with for example a GPU cooler or a further radiator. Without the reservoir it is not easy to fill the loop again.
But if that isn`t an issue for you, you can choose the LT version without any problems.