0758 Poole - Dorchester South 0845
This arrives Dorchester South 0832, Upwey 0840, Weymouth 0844.
Walk to Dorchester West
0903 Dorchester West - Castle Cary 0951
Dorchester South to West is very short & easy but if the OP does not wish to walk there is no need to, as this train originates at Weymouth 0850 and calls Upwey 0855.
Changing at Weymouth is possible, but not advised (I've done it with less time to spare though!) however changing at Upwey is fine, with 15 mins to cross from one platform to another.
That said, there my be additional costs changing at Upwey so perhaps Dorchester is better.
2 x £8.30 Anytime Single Poole - Dorchester South & Dorchester South - Poole
Tickets are issued to Dorchester Stations.
I was going to suggest a SVR from Poole to Thornford, as this is cheaper than 2 x SDS however the previously unrestricted ticket has had an 0833 restriction put on it by FGW (or maybe Wessex did that when they had the franchise).
An SVR from Poole to Yeovil Pen Mill is unrestricted from Poole and valid on the 0850 from Weymouth, priced at £17.80, £1.20 more than 2 x SDS for Poole-Dorchester but it is valid to Upwey & Weymouth so may be worth going for if the OP doesn't wish to walk in Dorchester.
£5.50 Std Advance Single Dorchester West - Exeter St Davids
£7.00 Std Advance Single Exeter St Davids - Dorchester West
Total £29.10
These are good value fares.