Changing grip costs an action. I don't see how you can use Sustaining Steel.

[Moppy]

CRB 272 Table 6-2

https://2e.aonprd.com/Rules.aspx?ID=194

strking spell, release 1 hand (free action), cast 2 action somatic spell, RE-GRIP two-hander (1 action), swing = 4 actions.

[MEATSHED]

You can cast a somatic spell while your hands are full.

[Djinn71]

Moppy wrote:

CRB 272 Table 6-2

https://2e.aonprd.com/Rules.aspx?ID=194

strking spell, release 1 hand (free action), cast 2 action somatic spell, RE-GRIP two-hander (1 action), swing = 4 actions.

You don't need a hand free to provide Somatic components, only Material ones (which are rare).

[Castilliano]

Somatic spells don't require a free hand in PF2 (unlike previously).
Only spells w/ material components do (generally the 3-action spells).

[Moppy]

Huh we had that wrong then.

CRB 303 confirms you're right

https://2e.aonprd.com/Rules.aspx?ID=283