Is there any way just to get the 105 Di2 shifters, calipers and the FD/RD derailleur only? You can cut weight going with an Ultegra cassette/crankset or go with an aftermarket setup.
Yep. I just picked the shit I wanted and bought them from all random places.
Merlin, Amazon, some other random joints. Worked out way cheaper to get separates. Was planning to mix/match shimano and ltwoo but it turns out ltwoo is utter garbage. Ended up with
105 fd/rd/crankset. Good cash savings, similar performance and weight. 105 rd has same internals as DA/ultegra. Have more faith in 105 cranks vs ultegra.
Ultegra shifters and cassette. Wanted top buttons n satellites without faffing with wires to a battery) and cassette has good weight savings for the cost. Servo wave is nice vs my 105 that doesn't have it, too, but whatever.
GRX brakes (same as 105/ultegra)
DA rotors (look coolest and same price)
Made my own satellite shifters for tops n drops for like 30bucks