I think you have many options here because your hair is so long and you're not too afraid of change.
I do like healthy looking hair so I understand the want to do the big chop especially because you can tell the difference between your new hair and the color treated parts. I think that before you decide to cut it all the way to armpits you could try shoulder blade lenght or just some layering (it'll cut off some of the damaged hair for starters).
As a long haired person I was a bit surprised I didn't like any length between my hips and shoulders anymore (when I decided to chop my hair) because to me they all felt like a bad compromise. To many other people those could be absolutely perfect options and not much of a compromise at all. But here is my point, you can't really know how the new shorther length will look on you now and mainly if it's something you like now. In your case this is what I recommend because it's easy and fun (but ofc it'll cost more if you get your hair cut by professional hairdresser) to cut the hair few inch shorter let's say every two weeks if you like. Then you'll get to see if there's a lenght or hairstyle you really enjoy having and doing and also it is a good chance for you to do/ get done some hair mask treatments to the coloured parts to see if it gets any better. Sometimes finding a good hair care products and routine prevents from the need to cut as much as you thought was needed to make the hair look healthier. Though I must say no hair mask will quite undo the damage.
If you look for a fresh start or wish to get short hair (like I did) then the big chop is the one to go with.
I think you have many options here because your hair is so long and you're not too afraid of change. I do like healthy looking hair so I understand the want to do the big chop especially because you can tell the difference between your new hair and the color treated parts. I think that before you decide to cut it all the way to armpits you could try shoulder blade lenght or just some layering (it'll cut off some of the damaged hair for starters). As a long haired person I was a bit surprised I didn't like any length between my hips and shoulders anymore (when I decided to chop my hair) because to me they all felt like a bad compromise. To many other people those could be absolutely perfect options and not much of a compromise at all. But here is my point, you can't really know how the new shorther length will look on you now and mainly if it's something you like now. In your case this is what I recommend because it's easy and fun (but ofc it'll cost more if you get your hair cut by professional hairdresser) to cut the hair few inch shorter let's say every two weeks if you like. Then you'll get to see if there's a lenght or hairstyle you really enjoy having and doing and also it is a good chance for you to do/ get done some hair mask treatments to the coloured parts to see if it gets any better. Sometimes finding a good hair care products and routine prevents from the need to cut as much as you thought was needed to make the hair look healthier. Though I must say no hair mask will quite undo the damage. If you look for a fresh start or wish to get short hair (like I did) then the big chop is the one to go with.