Unfortunately you don't have any simpler option. Here's a step by step explanation to make things easier for you. I see you already know how to activate developer mode so I'll skip that part.

So let's say you have three top menu items 'A', 'B', and 'C' that you consider should all be under one common menu called 'Alphabet'. Go into developer mode.

First you want to create the 'Alphabet' menu:

1. Go to 'Settings' > 'Technical' > 'User Interface' > 'Menu Items' and click 'Create'.

2. Set 'Menu' to "Alphabet" and hit 'Save'. Even if you do a browser refresh you still won't see this menu item as it doesn't do anything yet.

Now you want to put menu 'A' under 'Alphabet':

1. Go to 'Settings' > 'Technical' > 'User Interface' > 'Menu Items' and search for menu 'A'. Click into it.

2. Set 'Parent Menu' to 'Alphabet'.

3. Hard refresh your browser and now you should see 'Alphabet' as a top level menu item. From here you can click into it and see that 'A' is now a submenu.

Ok so now add menu's 'B' and 'C' as well.

Cool now the part about ordering menus: It's got to do with the 'Sequence' field when you edit a menu item as you did above. The sequence field tells you what priority level the given menu item has within it's parent menu (or top menu bar for top menu items). The tricky thing is that the sequence numbers need not start from 1 and step up by 1 at a time. Say you have menu items 'A', 'B', and 'C' with 'Sequence' numbers 3, 5, and 7824. Then Odoo just chooses the smallest number (highest priority) to show at the top (left) of the menu. Then it chooses the second smallest number and so on...

Let me know if this answers your question and if not I'd be happy to clarify :)