Just another one here to say that my wee one never drank as much as the tin said, never even close to it. Also he would never get into a 3 hour pattern the most he ever went between feeds was 2 hourly, this was on the breast and also when weaned at 3 months. He only went 3 hourly once solids were introduced. Some babies just need to feed little and often.
I expressed full time for 4 weeks and did half and half for about 6 weeks so it is doable but then my boobs went off like sprinklers for the pump or for Spencer or from me just thinking about feeding

Like Emz said though sometimes things can get in the way but I found the best thing to do when mixed feeding was to pump off the side Spencer wasn't feeding from and then when expressing full time I would pump every 3-4 hours and would try to do it when Spencer was asleep . I invested in a double pump as well so could express a whole feed out in 10 mins. Also I didn't dry up from expressing I stopped cos I kept getting infections so it is possible to keep pumping for long periods so long as you aren't infection prone.
With your plan I also agree with whoever said just let him have the week, don't introduce any top ups just yet and just see how you get on with the more relaxed attitude towards feeding. And don't panic about not getting much out expressing, some women just don't it doesn't mean that there is anything wrong with your supply.
If that doesn't work then I think your plan to give a formula feed before bed is a good one, that way he only has one bottle a day so boobie will still be the norm and he is less likely to reject you for the easier bottle and you never know a few weeks down the track you might be able to dump that bottle feed again.