Not having tubs in most rooms is fairly typical of most Greek hotels. Most Greek islands typically have water shortages (some even have to ship it in) so this may be an aspect.

The Domes of Corfu should be bookable by your travel dates, but the base rooms also don't have tubs and look to be smaller than the Domes Miramare.

https://www.marriott.com/hotels/trav...ph-collection/

The Mediterranean Sea will be pretty warm by August if that helps.

You are correct in your assessment of your upgrade chances in August.