First let me say that what you describe doesn't make you at all unusual. You say that you require a feeling of fullness and some G-spot stimulation to experience orgasm. Your partner's health issues make erections "fleeting," and yet you both enjoy intimacy; you'd like to explore adding a dildo or other device to increase your enjoyment.
Most couples find mutual satisfaction an invaluable component of intimacy; a partner's arousal and satisfaction enhances pleasure for most of us! Your partner's situation has changed, and if you're like most women, orgasm may have become less reliable over time, too. Now is the time to have a conversation with your partner—just to acknowledge the changes and that you'd like to try something new. (I wouldn't recommend that you produce a surprise vibrator or dildo during a intimate interlude! At that moment it could be taken as a judgment rather than an enhancement.)
The analogy we often use at MiddlesexMD is to vision. When it becomes more difficult to read the fine print, we get "cheaters" or reading glasses! We don't shelve our books. Why shouldn't we take the same attitude toward sex?
You may find that your partner is relieved to have you start the discussion. You might even look at options together and make the selection as part of extended foreplay.
Have confidence! It sounds like you and your partner have conquered more daunting obstacles together. I'm certain you can navigate this one.