Hi OP, we supplemented both of my kids; current little one was 2 years old at the start of this month and is still on the boob several times a day - whenever he gets the chance! :)
So in my experience combi did not shorten my BFing journey - in fact it extended and supported it, by removing the panic and fear I felt when his weight wasn't going the right direction.
Both of mine are fit as fleas and it did them no harm at all - in fact no doubt in my mind that formula helped massively when they needed to get weight up in the newborn days.
One of the big things I wanted when we began combi feeding was to feel less stressed - I felt I wasn't recovering as well as I could from birth due to feeding anxieties.
For that reason I did very much 'baby-led' combi feeding, didn't write anything down or do any maths - all of that fed my anxiety!
So I would offer the boobs, one after the other, and then after 10/15 mins if bub still seemed hungry I'd offer a reasonable-sized bottle of formula - 4oz or whatever seemed 'about right'. If bub chugged that down and still seemed hungry I'd offer boobs again and potentially make and offer a second bottle.
I'd always offer feeds whenever they seemed upset or hungry, never timed them or limited anything, but never woke them to feed either.
It's amazing when you stop hearing that 'hungry cry'! And they just get all relaxed and full and sleep really well... It did wonders for my post-natal sanity and recovery.
Wish you all the best <3