This build has taken me a bit longer mainly due to fit issues with the ER9 battery, but I finally solved the issue! I was almost about to give up and just build this bike mechanical or save up and get a Wheeltop EDS TX, but all my ideas pop-up in my head when I wake up in the morning!
Disregard anything I've written previously, these are the definitive steps to use the ER9/X on the r088 and possibly other sl8 clones! After examining the whole assembly, I found the full ER9 battery holder (with cover) can snugly fit into the seat tube, but in order to get it out, I tied 2 pieces of ribbon string to both ends of the battery holder. You can then easily pull it out for maintenance and/or battery replacement. The seat post will still need be cut, but no drilling is necessary for the Di2 clamp holder and you won't need to use it (only for the ER9/X). Though make sure to not lose it, replacement Di2 holders on AliEx are like $20 USD! The ER9 battery holder diameter is just wide enough that it won't shake around the seat tube. Though the fit is so tight there's no way to wrap it in either bubble wrap or even thin plastic wrap.
For both ER9 and Di2, I would definitely install the battery cabling first before installing the bottom bracket. I opted not to use the hole behind the FD cover. Routing the battery cable that way makes it extremely difficult (though not impossible) to get the excess cabling into the seat tube or else it will get in the way of the battery holder (the fit is very tight). You want to get all the excess cabling below the FD hanger mount in the seat tube. For ease of installation, I instead used the lower hole that could be used for mechanical. This is an option that I'm only aware of with the R088 and might not be available on the other SL8 clones/copies.
Anyways, this frame design really seems like a big f**k you to Shimano. Also the way the RD hole exits through the lower chain stay seems more like an afterthought. I definitely think this frame was build with full wireless in mind. Hopefully I can complete this build soon, since I was able to figure out this problem!