You can use nodebb-plugin-custom-pages to set up a new page and set it as the custom home page in the ACP.
To redirect users to a topic you can use the custom JS tab in the acp and check the users joindate and then redirect. For example:
$(window).on('action:ajaxify.end', () => {
if (ajaxify.data.template.topic && ajaxify.data.tid === 123) return;
const oneDayMs = 86400000;
if (app.user.uid && app.user.joindate > Date.now() - oneDayMs) {
ajaxify.go('/topic/123/welcome');
}
});
This will redirect anyone who joined in the past 24 hours to a topic. You have to update the url and topic id so the user isn't stuck in a redirect loop when they land on the welcome topic. Alternatively if you only want to redirect them once, you can set a flag in local storage and check before redirecting.