You could have the whole route every 30 minutes, with extra shorts running between the two P&R sites and then scrap the PR1 and save money there rather than having buses duplicating each other (this is how it works in York). Plus Doxford Park should be served all day imo, but it's just as quick to go via Silksworth and would open new links South.
The 20/20A would stay as it is now.