It's super easy to transfer to .com later on, but I'd do the transfer as soon as you know you want to commit to the website. .com's perform the best in search engines, so I wouldn't wait TOO long before making the switch.

You can always work on the free domain and then later transfer it to your own domain when you are ready.

Of course, your own domain would be the number one choice but only when you are ready.
Many have done well with the free domain

Transferring it over here is not an issue.

There is no need to wait for it to be indexed regardless of which domain you have or use the main thing is to follow all the training and apply what you have learned.

Keep writing great content and always go forward and the .com would be the better option again only when you are ready.

Wishing you well

Andre

If you can afford the .com I’d go straight to that. It is easy enough to move a siterubix to the new domain but if you are committed to making it work I’d just jump on the domain name now and get stuck into building your site.