@ACinATX Bridle Halter combos can be purchased at most endurance tack places like The Distance Depot and I think
@phantomhorse13 has a friend that makes beta tack.
Riding with a backpack makes your back sore and tired after a very short time no matter how lite the items inside of it. This is just going to be a short ride right? Try not to overthink it and not over prepare. If the park has a designated riding area the trails are most likely pretty groomed and in good shape. You may want to put a halter under your bridle for your first ride out but many many trail riders do not ride with a halter under the bridle (myself included) I feel like I know you from your posts and see that you are a person that likes to be educated and prepared but I think in this case it may take some of the joy out of driving your own truck and trailer to a nearby trail and just trying to enjoy. Remember you are doing this because you like riding your horse!
You can purchase a horse holster for your phone, put a small bag on the front of your english saddle and have plenty for your ride. I got my bag from Jeffers Equine and it fits the front of my saddle well and holds just a few things (hoofpick, small insect spray and any maps I need) I know Stateline Tack sells english pommel bags as well.
Have fun! We can't wait to hear how it goes.