The free space required by the Boot Camp Assistant to install Windows is larger than what is actually required. One question where this is documented is Mojave bootcamp assistant limits partition size. The answer to this question was to use the Disk Utility to create a partition for Windows, then install Windows the same way as any ordinary PC owner. The Boot Camp Assistant was only used to download the Windows Support Software.
In your case, I would first make sure all space being consumed by Time Machine is recovered. This can be done by executing the command given below in a Terminal Application Window. Basically, the command tries to reclaim any space taken by Time Machine by asking for a lot of space back. Once you have finished executing this command, go back and try using the Boot Camp Assistant to install Windows.
tmutil thinlocalsnapshots / 9999999999999
Other space issues related to installing Windows are documented in the accepted answer to the question: Installing Windows on MBP 2015 (Mojave) Using Boot Camp, Error: Not enough space!.