4. You can stack reservations for later in the day as long as you don't mind waiting
And speaking of reservation timing, though Disney allows those who use the $15 Genie+ plan to make new attraction reservations as soon as they redeem a return time, those who book a popular experience like Slinky Dog Dash first for a return time later in the day will be happy to know that they won't be locked out of making reservations for attractions while they wait. If you book an attraction for later in the day before the theme park opens, you'll be able to book another attraction 120 minutes after opening, and then can book another one after that one is redeemed (or wait another 120 minutes if you have another attraction late in the day).
So for instance, if you book Slinky Dog Dash at 8:00 AM for a 2:30 PM return time, you will have to wait until 11:00 PM (two hours after Disney's Hollywood Studios opens) to book a return time for another attraction. If you select an 11:30 AM return time for the Twilight Zone Tower of Terror and ride it immediately, you can then book a reservation for Toy Story Mania at 12:00 PM while still maintaining your 2:30 PM Slinky Dog Dash reservation.
5. Purchases should be made as early in the day as possible for full value
No matter whether you are interested in the $15 Genie+ service or the individual Lightning Lane attraction upcharge, you will get the most value out of these add-ons by purchasing early in the day. Technically guests can purchase the $15 Genie+ service at any point in the day (even 10 minutes before closing!), but if you haven't purchased this service within an hour of park opening, we'd say it probably isn't worth it as you'll be stuck with later return times. Similarly, the individual attraction passes are best purchased in the morning, as you'll have your pick of convenient return times to pick from. And considering you are limited to purchasing only two of these "a la carte" options per day, it is definitely in your best interests to secure the rides/times you want as soon as you can.
As with anything at Walt Disney World, we expect pricing and availability to change based on crowd levels and overall demand for this service. However, if you're thinking about purchasing this service, it definitely helps to have a game plan before hitting the park so you can get the full value of the Genie+ and Lightning Lane upgrades.