https://www.softprodigy.com/store/magento2/coming-soon-maintenance-mode - Whether launching or undergoing maintenance, retain your potential customers while you alter your website or store. Store maintenance mode comes to rescue when some change in the backend are being processed.

1. Store Maintenance-Coming soon

2. Coming Soon Magento® Extension
help store owners hold potential
customers with the coming soon
page. They put the front end of store
in maintenance mode while the store
is under construction. The extension
allows online merchants to display
countdown timer to notify regarding
the launch date and put the store in
maintenance mode for a specific
timespan.

3. Admin can preview the template before launch at fronte

9. After Configuration from backend for a
respective Page(Maintenance or coming
soon).Frontend will show with uploaded
respective images or count down timer.