If you want a flexible ticket, there's not much difference between a combination of Advance tickets and the relevant Off-Peak Return tickets. As you don't mind about the time of travel, you can easily travel within the Off-Peak time restrictions.
As an example, you could get:
Nottingham to Cheltenham Spa, Off-Peak Return ("SVR"), £33.45 with railcard discount, then
Cheltenham Spa to Bristol Parkway, SVR, £12.40 with railcard discount, then
Bristol Parkway to Exeter Central, SVR, £19.55 with railcard discount,
Total would then be £65.40 (return).
With advance tickets the cheapest total might be £63.10 (return), but would involve many more tickets - an SVR to Birmingham New Street, then an Advance to Cheltenham Spa, then another Advance to Bristol TM, another Advance to Exeter St David's, 2 Off-Peak Day Singles ("CDS") to/from Exeter Central, an Advance to Bristol TM, an Advance to Cheltenham Spa, and finally an Advance to Birmingham NS. Not sure that's worth it for the £2.30 saving, at least to me

, especially given that with the Advances you have to take the booked train or else get a new ticket.
There may well be cheaper combinations, so don't go buying just yet, but this is what a short play on TrainSplit.com has revealed! If you go with a single through ticket (an SVR) it's £78.15.
Also, be aware that there aren't any trains between Nottingham and Derby on the outbound journey - there are rail replacement bus services instead. It's included in the ticket, but obviously takes longer than if by train and is somewhat inconvenient - so if you can get to Derby yourself that might be better and cheaper.
EDIT: Just noticed I got the direction mixed up. Make sure you get it the right way around when you book (!), but I think the prices, tickets etc. would essentially remain the same.