Even though the R088 has mechanical cable routing it will be challenging. I would route all mechanical cables and hydraulic brakes first, before installing the bottom bracket. You'll need that BB space opening to fish the cable through. Also if you go with a regular bar setup with no internal routing, that should make things a bit easier. Even so, you'll still need to cut cables to size and you won't have much leeway if you decide to go with integrated bars in the future. In fact if you do go integrated later on, there's a chance you might have to redo the cable lines because they could be too short.

Trace Velo loves the LTwoo R9, but I'm not the biggest fan. First he never mentions what a PITA is to access the clamping bolts on LTwoo shifters. Also if you ever need to replace the shift cable, it's a very convoluted process compared to Shimano mechanical shifters. You can find Shimano 105/Ultegra components for good prices on AliEx if you shop around. It'll cost slightly more, but the ease of installation is so much better. If you go LTwoo, I'd opt for electronic. I've worked on both LTwoo and Shimano mechanical, modern Shimano mechanical is so much better to work on especially for a first build. Though I actually like LTwoo brake calipers. Despite the use of adapter plates, I find they have more leeway for adjust compared to Shimano's standard flat mount setup.

Anyways good luck!