@OP

I don't have too much to add at this time as I don't have a lot of time but I will say it would be best to attempt to try to keep a level head until we see the numbers on paper and can discuss the facts about it. I know that's easier said than done because WHM has been down this path before with a WHM vs AST debate but WHM still brings benefits to the table that AST did not back in 3.4 era when AST got overbuffed out the ying yang.

I know you said don't bring up Savage / Ultimate arguments only but I do feel it's an important aspect to consider because part of balancing revolves around the highest tiers of content because those represent kits being played at the highest skill tiers. Generally when you rebalance a kit you're either adjusting the skill floor or skill ceiling and the best place to look at skill ceilings are in the highest difficulties of the game.

As Miste pointed out in their comment, there is a rather large dearth of AST compared to WHM and SCH when you look at the FFLogs participants number. I know that one should take logs with a grain of salt but the participation numbers shouldn't be that divisive. I imagine S-E has seen this as well and are basing their adjustments to suit to increase utilization of AST.

My personal opinion at this point is the only thing that seems out of place is the Aspect Helios buff. I'm of opinion it's not really necessary unless the buff is a minor thing like a cast time adjustment. I don't think it needs a potency buff in either Diurnal or Nocturnal.

As a WHM main, the Lightspeed change to me doesn't feel too out of whack. Lightspeed never affected recast so all it did was front load the GCD. It'll be a DPS increase for Gravity in dungeon content and their wall to wall pulls. Lightspeed from a MP efficiency stand point will still be lacking compared to Thin Air since Thin Air is both 0MP cost and has a lower cooldown than Lightspeed (though the second bit is arguable due to Hyper Lightspeed trait).

The cast speed change feels like it's reducing cast time without changing recasts. This should allow for less clunky AST play and be a minor buff to their DPS as ASTs won't have to clip their GCDs as often. I'm okay with this though - from a high skill ceiling standpoint I feel this would give the AST about an additional 2 GCDs per minute (gut feeling, no theorycraft to back this up).

Anyway, that's all the time I have now. I hope you are feeling better now and let's try to maintain a level head until we get to see the changes officially on paper.