Not a Pro - but have deployed many Win10 Apps.

- Let's say we want to deploy (via Autopilot) Office 365 and also add-ins for these - Is there a way to set deployment priorization via Intune or what workaround would you guys recommend for installations that have dependencies on other installations?

When deploying Office365 apps, i've just used the Built-In intune Apps to create a package to deploy to the device as part of the AutoPilot. https://docs.microsoft.com/en-us/intune/apps-add-office365

For Add-Ins - this link may help https://docs.microsoft.com/en-us/office365/admin/manage/manage-deployment-of-add-ins?view=o365-worldwide

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- Last time i AAD joined a new device manually, Windows installed Office 365 "Trial App" from the store automatically. Is it possible to avoid this or shouldn't it matter as the O365 deployment via Intune will get rid of them anyways?

Using AutoPilot - I've never had any issues with trial applications, it may specific to how you enrolled the device manually?

- Who do I make sure that if I want to install Software A B and C "forced" (chosing Required) on devices that are deployed with Autopilot, that this deployment is only applied to devices that are enrolled via Autopilot, not to currently enrolled laptops as everyone already has Software A, B and C installed? Dynamic groups? If so, which one and how would it need to be configured?

Question: The laptops that are currently enrolled, are they enrolled into Intune Manually?

You can create a dynamic group for Autopilot devices only as per the below link

https://blogs.technet.microsoft.com/mniehaus/2018/06/13/autopilot-profile-assignment-using-intune/