Solid sun team; I can tell you've got some experience under your belt. I think your team is pretty cool, especially the use of Specs Heatran. It's an underrated threat, but no one ever wants to try it due to the prevalence of Rain. Since your team is pretty unique, I can't think of any profound changes. I do, however, think you could make more of Slowking's slot.
My suggestion for you would be to use LO TR Reuniclus. It's got the same typing, but it basically does the job Slowking is currently doing and does it a lot better. It will also give you a great second Trick Room Sweeper. It's a natural lure for stuff like Scizor, which you can take advantage of and get Heatran in. Many people rely on Tyranitar to beat Reuniclus, and you can take advantage of that in a big way as a Sun based team.
Before I go, I'll throw out Head Smash instead of Ice Shard. I think Ice Shard is pretty weak and generally useless. Head Smash is a cool option to nail Gengar and other Flying types that you would use Ice Shard on anyway. If you think Ice Shard is really important, I'd say keep it, but I find Head Smash pretty useful at times. If Donphan is at full health, it can also check Gyarados which you mentioned gave you a hard time.