If I started using pay as you go, would the maximum charge never be more than the price of the monthly travelcard
You can put your travelcard onto your Oyster card, which I would definitely recommend based on my PAYG experience (read 'rip off').
Case in point:
I went to Waterloo East and back the other evening using my Oyster card. This should cost £2 each way, but because the barriers were not working when I got back to Forest Hill (so I couldn't touch out), I have been charged a total of £6.90.
Sometimes the card reader says 'seek assistance' even though the barriers open and if you ask station staff, they say no problem and let you through - but unless you religiously check your card balance on line, you never really know what's going on. Because it takes 48 hours for your journey to appear on line, it's easy to lose track.
And if you do find a problem and need to call Oyster, it takes forever on an 0845 number. I spent half an hour on the phone to them - and they even called me back to resolve the issue - after I went in and out of a station without travelling anywhere and the system decided I was committing fraud and charged me the maximum fare.