Fix: Add property type declarations

[localheinz]

What is the reason for this PR?

• [x] adds property type declarations where possible

Author's checklist

• [x] Follow the Contribution Guide

Summary of changes

Review checklist

• [x] All checks have passed
• [ ] Changes are added to the CHANGELOG.md
• [ ] Changes are approved by maintainer

[localheinz]

Closing in favour of focussing on 2.0.0.