Thank you for posting your questions about building a pole barn-type structure. We have a pole barn blog article that covers the aspects of this type of construction. Here is the link in case you have not seen it yet. https://www.chiefarchitect.com/blog/post-frame-design/ If you follow this article, you should get wall framing without the top plate. If you are getting a top plate, you can always delete the framing manually if necessary.
I would recommend going down the path of creating a wall type with a layer for your horizontal wall girts (the strapping you referred to that the metal attaches to on the outside of the wall). This is covered in the blog article above. Once the framing is created, you can edit the top girt to the proper depth and thickness (2x8) and then make copies to make your 3-ply beam. Using the wall railing method is only better if you are making a building without metal siding (an open pole building with only a roof).
We have added additional functionality in our newer product (Pro 2025) that better handles the wall types used for pole barns. If you upgrade, you will have access to the new features and the help article above will better match the tools in the program. The price to upgrade Pro 2022 to 2025 is currently $395 (MSRP = $595). You can find more information about upgrading here. https://www.homedesignersoftware.com/whats-new/?from-version=23&from-product=pro&to-product=pro#purchase-widget-container
DerekPedersen's post in create wall with laminated 2x8 beam was marked as the answer