Formula does help with the relentless of feeding in those early days, though, let's face it. Someone else can help by giving feeds and though no one likes to say it, from what I can see with absolutely every baby I have had anything to do with it does seem to reduce the constancy of feeding at certain periods of the day.
I don't say this as a "ooh quick, whip out the formula now" thing, but because I think that pretending there is no time difference in the early days doesn't really help, as anyone with low supply or has to feed, say, two hourly who has peers with babies who are ff'd quickly see that actually, no, they don't spend as much time feeding. Even friends I had whose babies had reflux etc from formula STILL didn't spend as much time bfing as I did in those early days, though I won't say they were any less stressed or absorbed by feeding if they were having issues.
I loved breastfeeding, I did it until the eve of my son's second birthday, but the reality is that if you are looking at time inputted as a sole factor, when you bf you spend the first few months sitting under a baby a great degree of the time. You can do things to help, like sling the baby or work on one-handed feeding (I never found this easy personally as my baby would latch on and needed a lot of guidance) but you do have to accept that it is time-consuming. Later on, you almost gain this back because it is one less thing to get ready to get back and the baby's appetite (regardless of feeding method) is more stable etc, so bfing is far more convenient as at that stage the preparation for bottles is more of an issue timewise. Swings/roundabouts...
I think psychologically, you need to just drop the guilt rather than focus on the feeding method. A 20 month old is always going to be put out by the arrival of a new child, the new child is always going to take time away from them.. there is no way of changing this, whether you bf or ff.. the time you would gain from formula (which I know people will flame me for even suggesting is there to be gained) will not really change the fundamental facts of the shift in dynamic, and what you gain now, you lose later when arguably it can be more important (because the new baby is more interactive/mobile and requires more than feeding/patting/shushing/putting to bed in terms of time resources from you).
My grandmother bf'd 8 in a violent, abusive home where she would have had no help. By the time she had 5, the oldest was only just about to turn six. Despite having had just about the most catastrophic upbringing possible in many ways, they are all successful human beings with postgraduate education, decent jobs, long-term marriages (apart from the priest, I suppose!) and hey, they got through. You don't need to be in your 20 month old's face all the time, they won't break because you need to tend to your baby, they are learning valuable lessons about how the world doesn't revolve around them that toddlers the world over have had to learn. Don't be too hard on yourself.
Why it seems so attractive now is you just need a break. But I guess it's hard with five kids, it's only so possible for your dh to take over... are there other supports who could help out?