One question, could you please recommend any best practice approaches related to the Asset Inventory when it comes to SaaS accounts we have (shall those be listed in there or not, as we do not actually own the asset/account)?
In case these SaaS accounts are used to access or handle information that is part of the ISMS scope you are working on, the best approach would be for you to include them as outsourced services in your inventory. In case all these accounts refer to the same service, you only need to add a single register in your inventory (e.g., SaaS solutions for data storage, e-mail, collaborative software, etc.).
This article will provide you a further explanation about assets management: