@Fred Garvin , Looking for a quick solution with minimal time or experience. The Metashape is an excellent product and the Pro version includes tools to measure and additional features. Download the Pro version, and an email address will provide full functionality for 30 days. Another one is RealityCapture (RC) by Capturing Reality. They have full functionality version for Free, and you purchase PPI credits to export any final products desired... nice part, the credits apply to the photos, not the project. You can reuse the photos in multiple renderings for zero cost.
Both are great, but the needle swings to RC for full features for minimal cost... plus for charge-out per Client case... PPI credits you use, gives a 1 to 1 cost per Client with your mark-up.
All workable but still have extended time in learning and becoming proficient.
There are 2 ways to approach the measurements: neither are Survey Grade but that's not part of the focus of your question... more in a quick simplistic approach to accomplish a task at a low cost & quick to provide to Client.
To place accuracy of the measured point relative to the point on Earth, the need for GCP and GPS ground Rover is needed for accuracy... absolute. Although if you're wanting measurements of a section of yard, distance from house, or distance between points on land piece for Landscaping projects to calculate a project's cost, estimate material or provide a visual of plant placement. Not directly tied to an absolute points on Earth, but more accuracy in the size or distance between objects within the 2D Ortho. This requirement can have a small tolerance in accuracy... similar to tape measure in yard and probably better than GE satellite images. Then MapsMadeEasy (MME) would provide at zero or minimal cost and esentially zero experience beyond what you already have in experience to fly a grid autonomously.
The use of MapsMadeEasy, is a great way to provide these requests and provides the Client the ability to make multiple measurements, screen captures and prints. You simply upload photos from you MA (or new model), provide the Client the URL Link to your MME shared project of the 2D Ortho. Within Chrome the MME measuring tools are available. Make a MME "Location" and place multiple aerial dates into Location and Client can click through a 2D time lapse. You control the Shared MME links, and can close if desired too.
All quick, low cost (MME credits) and extremely quick to provide. If you shoot 350 photos, to see results, take every 3rd into a quick process batch and keep it under 250pts for Free Processing too. Let's you check results for free and it might even meet their needs too.
The measuring tools within MME are accurate to within a small tolerance within the 2D Ortho. Basically applying the math you were referencing above. Realistically, it doesn't matter if the landscape wall or shrub is spot accurate to the earth, as long as the length of wall or distance between shrubs is accurate.
And it's quick for you... your biggest time investment is flying the grid. The detail is much better than GE images, and it stays within your MME Acct if they later want a before/after comparison.