Your trip can be as planned or unplanned as much as you want. I am a planner by nature, so I love the planning aspect, and go completely overboard. My wife, on the other hand thinks I'm crazy, and has a more laid-back travel style. There is no one correct way to do things, but there are some things you need to keep in mind.
You need to have park reservations. This is why some planning is essential. Park reservations start to fill up a few weeks before and up to two months for busier times. MK and HS fill up fastest. You can no longer make a day of decision on which park you feel like going to in the morning. You can certainly book something and hop to another park at 2pm, but you need to have at least one reservation to a park that day. It is what it is. With your medical issue, it really is going to be a personal decision how you handle this.
Dining reservations open at 60 days. If you are onsite, you can book your entire trip starting at 60 days. This is what we refer to as 60+1, 60+2, etc. If you are offsite, you have to book each day, one day at a time, at 60 days. If you plan to dine in a park, you will need a park reservation. You can always book dining reservations closer to your trip, but you may not get the restaurants or times you want. Good advice is to book the day before because a lot of people are cancelling their reservations around that time. So, dining and park reservations go somewhat hand in hand. This is probably the minimum amount of planning you must do. It's not much different than it was before and isn't really complex. You should be good up to this point.
G+ makes things more complicated. Before, you still could plan your day, rides, shows, etc. There has always been ride strategies, which to go on first, which to book a G+/Fastpass, etc. This has not changed. What you see in the last 5-10 years are the
YouTube vloggers, message boards like these, and millions of other pieces of information on ride strategy. This isn't a G+ thing, it's a social media thing. People spend their life savings on trips to WDW and want to maximize their time and money, and social media has found a way to capitalize on this. Since there is so much information on strategy, it gets overwhelming. Ride strategy has always been around, G+ just made it more complex and harder to plan. If you want good G+ tutorials, check out the Earscouts and Mammoth Club YouTube channels. They are the best.
To answer your overall question:
You have to make park reservations in advance, no getting around it.
If you want to get the best dining reservations, your best bet is to book at 60 days.
If you want to maximize your time in the park, ride strategy is your friend. If you want to leisurely stroll around, that's fine too. It's just not as efficient.
If you plan to purchase G+, it's best to know how to use it for maximum effectiveness, otherwise you will be bitter paying for it and not getting what you expected.