Swansea City midfielder Cooper and striker Harris, who has made a fine start to the Championship season with promoted Oxford United, are called up for the first time in more than a year.
Winger Thomas also returns having started 2024-25 positively with Nantes after joining the French top-flight side on loan from Huddersfield.
Liverpool teenager Lewis Koumas retains his place having impressed in the opening weeks of a season-long loan at Stoke City, but Colwill, Fin Stevens, Joe Low, Charlie Crew, Charlie Savage and Matt Baker all drop out having been involved in Page’s final squad in June.
They are likely to feature for Wales Under-21s against Iceland on 10 September, with Matty Jones due to name his squad on Thursday.
Ipswich Town pair Wes Burns and Nathan Broadhead are absent for Turkey and Montenegro through injury, as is Bournemouth’s David Brooks.
Coventry City left-back Jay Dasilva misses out despite winning his first Wales caps in the summer, with the likes of Dylan Levitt and Tom Bradshaw also overlooked.
Wales: Danny Ward, Adam Davies, Karl Darlow, Ben Davies, Owen Beck, Joe Rodon, Chris Mepham, Ben Cabango, Neco Williams, Connor Roberts, Jordan James, Ethan Ampadu, Josh Sheehan, Aaron Ramsey, Ollie Cooper, Sorba Thomas, Kieffer Moore, Lewis Koumas, Brennan Johnson, Harry Wilson, Daniel James, Mark Harris, Liam Cullen, Rabbi Matondo
