I would go even further and make it so that the belt is tightened by some screw that moves the motor up and down. Additional plate that slides or sth similar.

As for how far the Z should extend is suggest you contemplate this screen snip and read post #8 where i am trying to explain the benefits of properly designed Z axis


The idea is to achieve maximum Z extension with minimum effort and maximum rigidity as a result, using the spindle body to make Z super stiff.
is more than enough for more


My advice is to understand this/about the Z/ and check your design backwards from there. 150mm Z travel is more than enough for all purposes.

The Z on the picture will extend 200mm with 3kw spindle , but that would seriously question a beginners design. As a precaution the plate should be 20mm steel or 30mm aluminum and reinforced with side braces /against front back deflection/ exactly where the black line is especially at the yellow area. Of course another design problems will arise from such an extension so you have to make sure left/ right you dont have any weeak element on gantry, gantry sides, ball screw mounts, etc.