The first time i heard about Sona's ult, I literally laughed out loud. And this is a great guide, and i hope to have enough IP to actually buy greater runes and not go dirt-broke.

This is true. I'm not really big on Ionian Boots of Ludicity, just because it's literally only useful to me for one ability with how short her other cooldowns are. The movement is much more important to me. Shurelya's Battlesong and Aegis of the Legion are both certainly viable options, I just prefer to play more offensively, I just find it useful to keep pressure on an enemy with damage. People get a lot less bold if they know that I can knock a good chunk of damage off of them while they're busy with my teammate. You do have a good point about levelling heal first, but to be honest, I usually allocate points situationally. If my lane partner's getting harassed a lot, heal takes precedence, but past that I prioritize damage for the reasons I said above.

You mention that Sona is one of the strongest supports, but you don't play her like a support at all. You should focus on gold/10 items early game and get the CDR cap by getting Ionian Boots of Lucidity + another 15% CDR item like Shurelya's Battlesong or by getting CDR boots, CDR Glyphs and Soul Shroud for your team.
Apart from that Aegis of the Legion is a really standard support item and I would level her heal first (I know this isn't your own skill order, but a support should focus on keeping teammates sustained, not on damage)...

The extra movement speed is super important, especially when escaping.