Congrats on finishing treatment and best wishes for the future!!!!
Not sure what decks are still open for your sailing but here are a couple of things that I might consider that may help you make your decision.
The biggest influence for me is that Grand Suites are Sky class and have access to Coastal Kitchen. You will have access to CK for the full day including breakfast, lunch and dinners. The coffee machine is available all day, you will have snacks and small bites and the lounge will have be open most of the day with the happy hour times in the early evening. Besides being a great place to hang out for the views and speaking with the concierge hosts you need to factor in how often you might be up there to get food, and drinks or for more formal meals.
Allure lounge/CK should be realtivily relaxed since it does have one of the lower "potential" crowds based on the number of suites/suite guests that can have access.
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The number of Pinnacles is the varying factor.
Unfortunately there are no GS on Deck 17 which would be ideal. (It is a great deck to stay.) Staying on a higher deck might be of value if you feel that you might use CK more often. If you are planning to do the Ultimate Dining package and/or the Deluxe Beverage Package then that area might be utilized less often so being closer to deck 17 is not as helpful. Deck 8 would get you closer to those specialty restaurants if you did have the UDP but usually only Chops and/or Giovanni's is open for lunch on sea days with the UDP. If you do have the UDP, Allure is only Oasis class ship with the Samba Grill. It is OK for a Brazilian Steakhouse but it is something that is not routinely on the other ships so it might be worth a try. That is in the Solarium in the evening so like Izumi, it is one of the few specialty restaurants that is not on Deck 8.
Finally the other thing is the activities that are on the lower decks. Deck 4, 5, and 6 would have easier access from 8 so factor in would you be doing more of the pools, sun bathing and solarium activities or heading to the gym, rock wall, skating rink, casino or the bars and restaurants on deck 5 and 6. The main theater, skating show and Aqua theater are all in that area so how much you are drawn to that and those other inside activities can influence your choice.
Glad to answer any specifics if you have questions.
Happy planning!