Hi Harshad,
It is easy to customize form submission thank you page. :)
There are two scenarios:
Scene 1: If marketing form is portal hosted, then we can add custom text and image by selecting whole form element and edit them in Properties section.

result:

Scene 2: If marketing form is self hosted on our own web server, then we can edit them in Marketing form > Form hosting > related marketing form pages:

result 1:

result 2, as you can see, we can also customize error message:

Moreover, we can apply further styling to elements on page:
e.g: Make a card to contain submission success text, image and button, increase default size of image, round the corner of reload button:

Due to these elements have specific class name which are generated by form-loader script, so we can add custom css style rules to overwrite them.
You can find name value of them via inspector with F12.

My style rules:
Add !important to overwrite default style.
Regards,
Clofly