It's smart to be careful with the heat and it is warming up in Portugal this week but I'm not sure it's heat wave territory yet. Max in Tomar today is 26 degrees Celsius (79 Fahrenheit), up to 31 (88) by Sunday. In Sep 2020 on the CP, we walked 12 straight days at 33 Celsius (91) or higher, up to 39 (102) in Azinhaga. Still, leaving early in the morning, and avoiding walking in the mid-late afternoon (the hottest time of the day) are good strategies.
Tomar to Coimbra is (in my view) the most interesting walking of the CP between Lisbon and Porto, with a lot of country walking among olive trees and vineyards. Coimbra to Porto had much more road walking and I found it a lot less interesting, so if you're looking to skip stages, that's where I would do it.
From Tomar to Coimbra these were our stages:
Tomar to Heart Way Pilgrim House (near Vila Verde but currently listed as closed on Gronze, ~21km/13mi)
Heart Way Pilgrim House to Ansião (~24.5km/15mi)
Ansião to Rabaçal (~19km/12mi)
Rabaçal to Cernache (~19km/12mi)
Cernache to Coimbra (~11km/7mi)
Check out
Gronze for accommodation options that could shorten these stages if they're too long.