The only problem with going in October is that they close the parks early to do the HHN ticketed events. If you are Disney people, you are used to that happening. There was a Halloween event last year, so unless the sky falls, there will be one this year.
Staying on-property, you will get all sorts of answers. The perks are early admission at all onsite hotels, and express passes for those staying at the premium hotels.
What the Harry Potter package gets you is a breakfast reservation at one of the HP restaurants. Pre-covid that wasn’t really a perk, since anyone could walk up and order. At the moment, it is a perk since they are the ONLY ones allowed breakfast. But, since you guys aren’t breakfast people … Also, by October things will have to loosen up.
Universal handles their ride reservations completely differently. Everyone has equal access to virtual lines and that is only on certain rides, and even then not all the time. It is used on rides that don’t have enough queue real estate to hold the social distanced lines for it. It runs similar the Boarding Groups at Disney. You get the virtual line reservation the day of.
People compare Express Passes to Fast Passes. Think of an Express Pass as having a fast pass for EVERY ride and EVERY time period. So, there is nothing to reserve ahead of time. You can buy express passes. The price varies by day but probably $200 per day per person. They don’t want lots of people to have them because otherwise the express pass line will be like standby.
Or you can stay in one of the three premier hotels and get express passes included for every person in the room starting from when you check-in to the end of checkout day.
Since you will only be there for two+ days, I’d stay at least one night at a premier. It will be crowded. Even in low crowd times, weekends are crowded. I would book the premier room NOW and then watch for specials, in which case you can change to the new rate. I don’t normally use TA’s. But I’d get one for this reservation. But, during this Spring Break prices went up and up and up. I am really cheap, so I’d stay one night offsite at someplace place cheap on your arrival night. Go to the parks and just explore on arrival day. Then check into you premier hotel before early entry, get your keys and enjoy the parks.
The reason for using a Liner TA is that they can watch for passholder rates for your hotel room. For example, a premier can be 260-ish after the passholder rate. You will need to churn your numbers but it is often worthwhile to upgrade one person to a pass in order to get the passholder rate. You can upgrade a ticket while you are there as long as it isn’t used up.