If a piece of work requires manual testing/QA, do we need to wait until opening PR?
The guidance that we've always followed in my various scrum teams is developers should get what they've got into testing as soon as possible. It may mean a little rework later on down the line if there are issues found in code reviews or developers haven't completely finished coding, but the earlier developers get feedback on what's been done, the earlier they can fix any issues, and take account of this feedback in any further coding. 

You should make testers aware when they aren't getting completely finished output, but if they've been encouraged to think about the benefits of preventing bugs as well as just finding them, they should hopefully be keen to adopt this process. 

It isn't always practicable - for example, if you don't have deployment tools that are flexible in which branches they deploy from, but when it is practicable, it's what Scrum and agile are all about - get feedback and get it early.