The Montgomery is lovely and IMHO worth the trip. Llangollen to Frankton is about a short-ish day. You must book Frankton locks, hire base will tell you how - you might need to do this online rather than on the phone, so take a mobile/tablet. You must book not later than the day before you go down, you book both down and back up at the same time, and you must spend at least one night on the Montgomery, but unless you want to race to the end and back two is better. You don’t need to book the other locks on the Montgomery, or New Marton locks.
Having come back up, Ellesmere is only a couple hours, a nice town with the most convenient Tesco you could ask for. Go on a bit further and the moorings alongside Blakemere are very good.
If you like pottering that would probably be enough, if you like boating dawn to dusk you could go further, but although Whixall Moss is ecologically interesting, there’s not a lot between Ellesmere and Grindley Brook. Going down and up Grindley Brook could take you all day if it’s busy and there are queues; I can’t remember where the next winding hole is below Grindley Brook, I’ve a feeling it could be Wrenbury.