Is there any other way to access the upper abdomen to repair the DR?

No, you would have reoperate to have access to the upper abdomen and tighten the upper abdominal rectus fascia to the midline. Having left the upper diastasis untightened and pulling in the bottom just makes the bulge worse and is a possible outcome even if the upper and lower diastasis are repaired at the same time, as they should be. Gauging how tight to pull the muscles together, particularly the upper abdomen is a problem, compounded by the fact that the patient is lying supine during surgery, but any uncorrected laxity in the upper abdomen may not be apparent until later when the patient is in the standing position. There are various techniques to avoid this outcome, but if the upper abdominal bulge was not uncorrectable due to intra-abdominal fat, then the treatment would be to re-operate, and through the tummy tuck incision.

Without in person consultation or, at least, series of photos, it is very difficult to comment on this problem. Unfortunately, if diastasis repair is indicated, the best approach would be through an existing abdominal scar. However, not the entire incision/scar needs to be used for the approach and repair. Good luck.

In order to access the upper abdomen your surgeon can use your existing tummy tuck scar. This way she/he can address the correction above the belly button as well as make sure all looks good from the previous repair. She/He can also access it from the top of the abdomen but this will involve another long incision extending from under one breast to the other. The best way is probably the way your surgeon is suggesting.

If your surgeon uses the scope, it could be done through small incision, otherwise you will need to opened to get access to it.

Sadly, the only way to plicate the rectus fascia is to incise the skin. You must either open the lower abdomen as in an abdominoplasty incision or a vertical midline incision (not aesthetically acceptable ).
Why didn’t your surgeon plicate The entire rectus fascia originally?

Your surgeons decision to not tighten the muscle wall from the rib cage to the pubic bone is unfortunate.
I’m guessing did decision may have been based on lack of experience.
What your surgeon is proposing is the correct approach and it makes no sense to add a second incision and give you two scars. You could conceivably get access to the upper abdomen through a reverse tummy tuck incision but that would just give you double the amount of scars.The recovery is the same. What makes the recovery difficult from a tummy tuck is not the incision but the muscle tightening part.
Skin only tummy tuck has an easy recovery in comparison.
Granted, patients still have to manage drains which can be frustrating but what really sets the recovery from a tummy tuck apart from all other cosmetic surgical procedures is the intensity from recovering specifically from the muscle tightening.
It’s unfortunate you need to have the procedure done twice to get the results you want but the correct approach is going through your old incision.
There are general surgeons who try to do laparoscopic muscle tightening but I have yet to see results that look impressive with that approach.
While it’s unfortunate I think your surgeon is giving you the correct recommendation.
Best,
Mats Hagstrom MD