SEO -Addon domain.com targeting domain.com.au/public_html vs 301 redirect using htaccess

I'm experimenting with Addon domains.

I understand that normally one establishes a redirection 301 when it points, for example, domain.com in domain.com.au (where domain.com.au will become the new main domain with a new website)

The old domain.com has a wix website that we are discarding and moving everything to a new hosting service in a Windy control of Centos Web Panel.

I was considering

set up two copies of the website (ie domain.com and domain.com.au) and host them in separate accounts, OR

Configure the new website in domain.com.au and create an add-on for domain.com with redirects using htaccess.

I am using Centos-web Panel. I notice that when I create the Addon domain, I have the option to set its home directory.

So, if I set the home directory of the additional domain in domain.com.au/public_html (where the new wordpress website resides for domain.com.au), is that good enough for SEO rankings?

Essentially, if I now write domain.com, load domain.com.au. Will it work or does Google penalize a change from .com to .com.au even if the "domain name" is the same?

What negative aspects and other problems can I expect from doing this?