As pointed out, the disc is attached to the hub, so wheel flex which involves the rim and spokes is immaterial. OTOH, of there's any play in the bearings, the hub could deflect slightly under load.
But disc rub has a particular chirp, and I strongly doubt you could have rubbing material enough that you'd feel the friction without hearing it also.
I suspect that the bike is fine, and it's all illusory. You're probably feeling the reason you're standing and riding slow in the first place. Sometimes hills are steeper than they look.
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