Hawaii..Plan our own or use travel agent?

Are u wanting hotels or condo rental? I would say most people arrange on their own. You can check out Booking.com or do like me and google vacation rentals! I lived there for several years, if you give me a little more information I would be happy to help. I even know many of the hotels and vacation rentals as I worked in tourism while living there.
You could check Costco.com for package deals.

Costco has package deals to Hawaii.

You can look at booking.com or just do a search for 'rental condos( name of island)' and get lots of choices. Then its a a matter of sorting by budget, locale and in your case accessibility. Many rental condos were built before ADA and have no elevators, so 2nd and 3rd floor access is via stairs. Hotels will, in almost all instances, have elevators.

If you plan to do a 2nd island, I would suggest go to Oahu 1st, spend 4 nights and then go to your 2nd island and fly home from there.

Aloha, you will enjoy it.

Normally I suggest that people handle the planning themselves, but given your mobility issues I think that working with an experienced (and hopefully knowledgeable) travel agent might well be the best fit for you. Suggest finding someone locally so that you're able to work with them face to face rather than over the phone. A good agent should be able to craft an all inclusive package that includes handicap accessible day tours that will preclude your having to worry about getting a car.
For a second island I suggest that you consider the Big Island rather than Maui or even Kauai (where we used to live). My qualitative opinion is that there are more handicap friendly activities for you there... the flower farms of Hilo, Kilauea Volcano, the beautiful beaches on the Kona side and the cowboy country of Waimea. There's a lot less traffic on the Big Island which will make for a much more mellow experience for you - especially after the hustle and bustle of Oahu.
Have your travel agent try to find you a good deal on an open jaw flight plan, ie flying into Honolulu, thence an interisland flight over to Kona, and then home from there. Will save you having to backtrack in order to depart from Oahu.
Given a choice try to go in May versus June. May is still shoulder season in Hawaii so prices ought to be a bit better than June when the flood gates open for Hawaii tourism.

I'd suggest using a travel agent this time, especially as it's your first trip and you have mobility issues. The travel agent will be able to book a package deal that includes flights, transfers and local tours.

Hopefully one of the forum members from Virginia can recommend a good travel agent in your area.

Thank y'all so very much. All very helpful advice.

I’ve chatted with a number of travel agents over many years and only a couple had a decent foundation in actual travel.

Of course we did use travel agents a long time ago but for many years now have done all our own booking, despite having a travel agent friend. (oh, the guilt!) However, late last year we got lazy and had our travel agent friend book our flights out west. The day we were to return my sister was unwell so we phoned the T.A. friend in Montreal and within 5 minutes she cancelled that day’s flight, rebooked us for 3 days later and pursuaded Air Canada to forego the penalty charge. In times of need .......

The few remaining travel agents near me are going the way of buggy whips as their market is predominantly elderly folks booking cruises.

I'd move over to TripAdvisor and ask over there. Many there will tell you it's easy DIY and many will have advice about mobility.