I had the same issues, I have less than 800 items on my list, and i did the following:
1. I used dropdown, but it only displayed about 600 items
2. I adjusted the limit to 2000, then it said "maximum number of items reached" at the bottom of the list.
3. I changed it into a combo box (single selection only), now all items are visible in the list, but when I click my save button with formula SubmitForm(EditCreateForm);Navigate(GalleryScreen,ScreenTransition.None), it didn't navigate to the GalleryScreen. I cancelled it, but then I saw the data was actually saved.
4. I selected the item and tried to edit it. When it went to the edit screen, the combo box was reset. But when I go back to the display item screen, the data is still there.
5. I have been trying to put DefaultselectedItems (e.g. Parent.Default, Lookups,etc), but it keeps giving me an error saying "The property expects table values, but this rule produces incompatible number values"
So looks like I have 2 problems:
1. EditForm save button doesn't navigate to the right screen when selected. From what I gathered, this might be caused by the combo box being in the form.
2. Combo box default selected item needs to be set.