Would you consider starting IVIG as initial steroid-sparing agent instead of azathioprine/methotrexate in a patient with inflammatory myopathy, high Jo-1, weakness with severe dysphagia and without skin involvement or ILD?

Yes, that would be one approach especially given severe dysphagia. The best treatment for dysphagia is IVIG.
However, steroid + MTX/AZA would be a reasonable approach for 2-3 months. If not seeing desirable results, going for IVIG early rather than late would be a good strategy as well.