As for your tank perimeters, I think the temperature is a little high but although it could possibly have made the angel more susceptible I do not think it was the cause. I believe the recommended temp is 25 and have read that higher temps can shorten the life span of Angels.

Personally I would split the water changes to 2 35% changes per week rather than doing one large 70%. Reason being, I would personally be afraid of removing too much beneficial bacteria with a 70% change. This would cause your Ammonia/Nitrite to rise until the biological cycle could catch up. You are probably okay but I would worry. I know a lot of people who keep discus though do a lot more and typically larger water changes so this may be okay.

I would also recommend getting a Nitrite and test and one that does both gh an kh. These 2 test imo are all you really need and can help you see issues with your tank quicker. Any time I see one of my fish that does not seem right I check for Nitrites, yeah the issue could be ammonia but with high ammonia I would end up with high nitrite as well and imo it is more deadly. A lot of fish stores with test you water for you but a lot do not commonly do anything other that ammonia/nitrite and ph.

Although Angles, Discuss and corries like a low gh/kh but really low (zero) of both or either can negatively effect your fish in the long term. Grant (Island Pets) has a thread (or two) in there sponsor section with a lot of details on this. I have a lot of fish that prefer the soft acidic water and used to loose a few every now and then until I started to maintain my gh/kh both between 5-8. Since then I have not lost a single fish.

Before you rush to drop your temp or add anything drastic read up on the swim bladder disease. At this point I think the main thing is treatment, which unfortunately probably means at this meds at this point. Once the fish recovers you can make the tank changes (if you feel they are necessary) if you do them now it may just stress out the fish, which is already weak and may cause more damage.