You can call collections and ask for a freeze of your DD at a lower level or a return of some of your credit just before your switch.
0800 0699 831
The rules for refunds after switching are the same for all suppliers as you probably already know.
Up to 6 weeks for a final bill. Then up to the 10 days for any credit refund following the final bill.
https://www.ovoenergy.com/help/leaving-ovo
These are industry standard times that all suppliers have to meet.
Miss either of them and £30 compensation is normally automatically due.
Obviously depends on what you want from a supplier in terms of tariff etc , the Which report is worth a look at, although not perfect
https://www.which.co.uk/reviews/energy-companies/article/best-and-worst-energy-companies/which-energy-survey-results-ajqM43e6ycY8
Trustpilot is another useful proxy, the big suppliers left for example are
OVO
https://uk.trustpilot.com/review/www.ovoenergy.com
EDF
https://uk.trustpilot.com/review/www.edfenergy.com
Octopus
https://uk.trustpilot.com/review/octopus.energy
Eon Next
https://uk.trustpilot.com/review/eonnext.com
British Gas
https://uk.trustpilot.com/review/www.britishgas.co.uk
Scottish Power
https://uk.trustpilot.com/review/www.scottishpower.co.uk
Shell
https://uk.trustpilot.com/review/shellenergy.co.uk
Utility Warehouse
https://uk.trustpilot.com/review/uw.co.uk