I guess your options are to deal with a real estate firm or try to find an owner (or owners) who has what you need and rent directly from them.
Rehoboth has quite a few large houses and also some properties that have several separate buildings on the same lot.
I know a large family who always vacationed in three adjacent cottages in Dewey Beach, which is almost an extension of Rehoboth and has plenty of restaurants and all the "action" you could possibly want. It is a younger, lively crowd, Rehoboth is more "upscale" with specialty shops and many varied fine dining options.
Dewey is more densely populated, hence it has more affordable "beach cottages" within a block or two of the beach than Rehoboth. But finding a pool in Dewey could be a challenge,
Good thing you have plenty of time to research this.