Once you've introduced solids, you are doing mix feeding anyway IYSWIM.
It's totally OK to pick whichever feeds you want to BF or not at.
In those early days when you were EBF, it was important to feed to keep your supply up, as it was the only food your LO was getting.
But now you're introducing solids, your baby is getting sustenance from several sources anyway, not just milk. And if you give - say porridge with milk or yoghurt as food, why not FF from time to time also?
What we did was I BF DS when I was there, but on occasions when I was away and DS needed milk, DP fed him formula. We never used bottles at all, he was maybe 9 months I think when we started doing that occasionally (my memory is hazy, sorry!) and we used a beaker or sippy cup IIRC. (So the good news is you can go shopping / to the cafe without having to quit BFing!) We didn't use the powder milk, we used the pre-made cartons you can get. You wouldn't want to use them all the time as it'd be pricey to do that if you were FFing regularly, but for the odd feed, it's so much easier than faffing about with powder and bottles IMO. If you're worried about nipple confusion, maybe you could try a beaker? As it's such a different action, maybe less likely to confuse IYSWIM? Although at this stage, you and your DS are BFing pros, I would imagine nipple confusion is much less of an issue now than in the early days.
One thing about BFing, I think we do it a disservice to talk of it as just a food as it's so much more than that. It's like an amazing supplement, tailored specifically for your child. I expect you're well aware that BFing supports your LO's immune system, but do you know how that works? Your body is exposed to any infections your baby might have, through general contact, but also specifically, through contact with your nipple when breastfeeding. Your body then makes anti-bodies tailored to what your individual baby needs, and your baby receives them in your milk. Your boobs are like your baby's own personal pharmacy :)
BFing is also a way you can give comfort to your LO - and that's important. So often people say "s/he just wants comfort as if that's a bad thing.
In this context, if you are BFing, I would say it doesn't matter so much when you feed - if you need to change your feeds around one week (when you're off for a shopping trip for your lovely new dress, via the cafe :) ) and your baby copes fine with that (most do IME) then don't worry too much about sticking to a routine, necessarily. To make the most of breastmilk, it's more important that your LO is getting feeds, rather than exactly when he's getting them IYSWIM.
It would be absolutely fine to feed 5 times one day, then substitute FF for a feed when you're not there another day, if that suits. Your body will adapt. You don't necessarily need to go down to 4 BFs a day and one FF every day IYSWIM, you can adapt to whatever suits from day to day.
FWIW I am currently demand-feeding my 15 month old DD. Three days a week I'm out of the house at work, and she gets water from the CM. Then the rest of the time she feeds during the day too, again on demand, so much more those days I'm home. My body has just adapted to that, no problem.