Aiyoo sakit lengan memang relatable! Honestly a few of us tried switching to sippy cups or open cups around 6 months instead of going back to bottles, since baby already got used to direct latching, sometimes forcing the bottle teat again boleh jadi struggle. For formula, mixing with breastmilk is perfectly fine at this stage, lots of mums I know do combo feeding without issues, so follow what works best for your supply and baby's needs lah. Maybe try those wide-neck bottles or even just cup feeding for formula while you continue expressing, your arms will thank you!