For IHA's we have found that adopting Agile principles for different tiers of projects has been the best way for teams to embrace it. What that looks like from a high-level is new concept creative (tier 1) works very well in an Agile Scrum approach, production level work (tier 3) works very well in a Kanban approach, and repurposing/adoption of existing work (tier 2) tends to do better in a hybrid variation between waterfall and Agile. So maybe the team does daily standups to understand progress but doesn't work in full sprints.
We actually published a whitepaper on this topic that you can download for free here. https://cellainc.com/insights/whitepapers/2020/01/20/making-agile-work-house-agencies/.
Hope this helps gets the creative juices flowing!