I was in your shoes like 4-5 years ago, I was working on SQL Server, SSIS and SSRS back then, and i was given a Power BI opportunity and then Power Apps.
As you are a Power BI developer, It will be smooth addition for you, In my view there is no particular order or anything as such.
I would suggest using Microsoft learn, Use the "prepare for exam" sections in PL-900, PL-200 etc certification pages, In this way you will learn every tool step by step. As of Co-Pilot, you can learn it using Copilot Studio Agent Academy
https://microsoft.github.io/agent-academy/ (hands on)
For more structured courses, you can take a look at these
As you mentioned, Even Power Platform guys are not always tied to an IT department, Like if you take my example, I used to work for a IT services company as a Senior Software engineer, now I am more on business side as a Solutions Architect for HR department of a Hospital Chain, I work independently and exclusively for HR not tied to IT. It mostly depends on the job role and level of solutions that client or company needs.
No one can predict the future, but Power Platforms is rapidly evolving as per market, Code Apps just got generally available and there is vibe.powerapps.com and many firms are adopting power platforms.