Grooming difficulties

Take her to a professional groomer and have her groomed by them. That way when you start to work on desensitizing her to being groomed you have longer to work with her without any knots.

You need to break up each step to brushing her and pair each with a very high value treat. Work each step till she's comfortable with it before moving on to the next one, that's why I say to get her groomed first.

An example of how to do it would be having the brush on the floor and giving her a treat for smelling it, repeat for however long it will take to have her happy to see the brush, it may be a couple days, it may be a week. Next might be picking up the brush, giving her a treat, and putting the brush down, again repeating for however long is needed. Next try touching her with the brush, giving her a treat and removing the brush. Then try one stroke with the brush. Next a few strokes. Again always pair it with those yummy treats.

Make sure to break the treats up into very small pieces. That will allow you to do longer sessions and not have him gain weight. It also makes it easier to do jackpots, where the dog gets a bunch of treats when it does something really well.

You might want to ask your groomer how she's been for grooming, and if they have any tips, as they work with lots of dogs, and likely have suggestions that will lessen the stress on both you and her.

Your groomer probably has her in a grooming noose on a table, which limits their ability to get away/turn around, etc. You may try standing her on a low table or bench to encourage her to stay put, or tie her with a slip/quick release knot to keep her still on the ground. This will eliminate the running/calling/ignoring part of it, but you will still have a dog who is not enjoying the process. There are detangling sprays which can be used on mildly matted areas, and you can probably have your groomer show you how to safely cut/clip mats out if they are beyond the help of the spray/brush. If it is uncomfortable, she'll never come to enjoy it.

To recondition her to enjoy being groomed, you can try a smear of peanut butter or cheez whiz on the table or wall near the table to distract/keep her busy, and start with just a couple brush swipes, working up to more, and giving the reward later in the session. Eventually, you'll be able to brush, and just give the reward at the end, or if you get to a spot which will be tedious.

If she doesn't like standing/being held in place, I would condition her to that first, as the easiest way to keep them from sitting when brushing their butt/hind legs is to just put your hand under them, but if it makes her uncomfortable, you have to work on that first. Avoid physically wrestling with her though, as that's not fun for either of you.