I have a minor correction.
You describe the level 10 Inexorable Iron Feat "Sustaining Steel" as:
"When you spellstrike using one of your spell-slots, you regain hit points equal to double the rank of the spell! "
This isn't quite accurate. You have added a restriction while missing the one that is present.
Sustaining Steel requires you to be in Arcane Cascade stance, but it triggers off any spell cast from a spell slot, making it much easier to use.
https://2e.aonprd.com/Feats.aspx?ID=2874
The overall amount of HP gained is rather low, so it is still not the most optimal Feat, but it essentially gives you free HP whenever you spend a resource you were always going to spend. Whether it is a buff spell or an attack spell. And, importantly, it allows you to use spll slots gained via archetype.
I find defensive reaction spells an excellent trigger, taking a frankly unhealthy amount of pleasure from using Wooden Double to make an HP profit when an enemy crits me.
https://2e.aonprd.com/Spells.aspx?ID=1416
As you rightly point out, the small amounts of HP provided by Sustaining Steel combine well with the Temp HP an II gets from Arcane Cascade, it can be fun to also throw in Mantle of the Unwavering Heart, picking the fast healing option.
https://2e.aonprd.com/Spells.aspx?ID=1406
You can immediately regain 10HP from casting the spell, gain Fast Healing 3, and then every turn gain temp HP.