All 3 of mine had/have occasional bottles of ebm.
I spent a few weeks expressing after the first morning feed (end of side fed off then all of the other side). I used to feed, then express before getting out of bed and it just became part of the routine. I also expressed off the opposite side to the last feed before bedtime. When I wanted to reduce the amount of expressing I gradually expressed less and less each time down to about 2oz then stopped, never had any problems with engorgement or being uncomfortable.
I had a fantastic stock until DH accidentally left the freezer door open overnight and I had to throw a whole freezer compartment worth of milk away. He was not popular.
So, I am now back to expressing in the mornings again for dd (6 months).
If she does have a bottle for any reason, I either express off that feed and freeze it as soon as I get back, or feed her from the side that she missed the feed off. I don't find missing one feed causes any discomfort, any more than one and I start to feel fuzzy headed and uncomfortable though.
I use born free glass bottles, which have suited dd fine.
I did give both my dss and dd a bottle before 6 weeks, as I had heard they are more likely to accept a bottle that way, but 9 weeks is not that far off so you should be fine.
The only thing I would say is that some HVs tend to recommend you don't start expressing until after 12 weeks so that breastfeeding is fully established. I started expressing at 6 weeks with all mine though and its been fine with all 3.
I particularly wanted to make sure I had a good freezer supply for dd, as with both dss I was admitted to hospital suddenly and had to give up breastfeeding overnight. Having a good stock of ebm in the freezer meant they could be switched to formula gradually and made the whole process less stressful.