Be sure to contact support directly, and let them know if you want this as feature request.
The keyword for this is: transactional email ; as opposed to marketing emails.
You probably need a CRM with email flowing through that as it can solve a lot of headaches longterm.
This exact mix of wants is a unicorn app, for instance apps cannot modify the admins customer contact form, that’s something that would require browser userscripts or a browser addon/extension.
You’d probably have to build something custom to get close, or hobble it together from the below ingredients:
Some email services have some sort of boilerplate/template functionality if customers are emailing your inbox (e.g. gmail).
Or maybe shopify inbox which has quick replies
https://help.shopify.com/en/manual/inbox/configure-inbox/quick-replies
If there’s a lot of copy&pasting going on look into either clipboard/clipboard-sharing apps, or text-snippet apps/services.
Most apps that add an entry to the admin actions may only show a generic entry i.e. “Reply to customer in App X”, so staff would go to the customer admin find the customer then pick the apps action , navigate to the app, wait for app to load , then navigate to and pick the response boilerplate to send.
So that’s something to watch out for as a subfeature that it is somehow a 1-2 click action and not more actions than if you have just copied & pasted.
There are automation apps like firepush, mesa,mechanic, to build workflows like this.
Like with mechanic you’d script some transactional emails , then in the customer admin choose the admin-action-link “send to mechanic”; Or build a custom dashboard to trigger workflows as runlinks probably as bookmarklets or browser extensions.
https://learn.mechanic.dev/core/shopify/admin-action-links
Another approach is while there’s no timeline is to poll* customers data and check for new timeline comments, so staff could add a comment “send out of stock email” and an automation app reads that to trigger that specific email.
https://learn.mechanic.dev/faq/whats-possible-with-timeline-comments#whats-possible
There’s also depending on your setup using an automation service to reply automatically based on keywords.
Good Hunting.
*Currently timelines do not create api events or have webhooks , so data just have to be constantly checked if you try to react to timelines.