There has been an improvement in accuracy in ASA SDK 2.10.0 - This release of Azure Spatial Anchors SDK includes a change that improves the accuracy of spatial anchors by leveraging the state of device's native tracking system in a better way. While locating existing anchors created with older SDK versions will improve, the most improvement comes when new anchors are created and located with this new SDK version.
That being said, I cannot tell you how significant it is for your use case and it's ultimately something that you can best determine after running some tests. However, the recommended Unity version is 2020.3 LTS, especially since 2019 is going to run out of support soon. Additionally, we are not recommending Unity 2019 due to Azure Spatial Anchors incompatibilities with AR Foundation 2. It is also worth pointing out that there was a timespan of 15 months between ASA SDK version 2.8.1 and the current latest version, 2.13.0, and there were a lot of bug fixes (see release notes) which are not backported. In that sense, we're always improving our products and generally suggest using the latest versions.