I've never driven one, but we travel with DD in her manual wheelchair to WDW. She drives her power wheelchair whenever we are out of the house at home. As for being in people's way at stores, we have as much right to be there as anyone else. If an
ecv is what you need to be comfortable, you should not feel that you are an inconvenience to anyone else. Other people do lots of un-useful things that inconvenience me (like park their cart smack in the middle of the aisle), so I don't feel at all bad to maybe inconvenience someone a little when we are doing something useful. Also, anyone who feels a wheelchair or ecv is inconveniencing them, needs to get a life. They could easily end up needing one someday.
IMHO, Epcot would be the best place to start with driving an ecv. You will see a lot of them there, the walkways are wider in general there and the crowds are more spread out. MGM is pretty good too. After you are comfortable there, you can branch out to parks with tighter layout like MK and AK. Study maps so you are confident about where you are going.
Do yourself a big favor and rent one. My FIL has been saying for the past 10 years that he will rent "one of those carts". This year, again, he didn't rent one and he was in a lot of pain. It makes us feel very sad to see him struggling so much and it limited what he could do while on vacation. They are expensive, but the cost to your body is "expensive" in energy too.