Opening limits aren’t issued randomly, there issued under the guidance of FAA order 8130.2J. If you want to see the differences between S-LSA, E-LSA and E-AB download a copy
https://www.faa.gov/documentlibrary/media/order/faa_order_8130.2j.pdf
have a look at appendix D and build yourself a set of each.
It’s not the DAR, it’s the FAA that allows IFR/IMC operations though the opening limits. The airspace we fly is the FAAs domain, ASTM has no jurisdiction
Look at the op-limits, compliance to operations dictated in the Manufacture supplied POH only applies to an S-LSA. If your an E-LSA add the required equipment and modify the page after certification, after all you are experimental
An E-LSA only needs to match its S-LSA duplicate at the time of certification. Once certified you can modify it all you want as long as you keep it within the definition of an LSA (FAR section 1.1) and follow your op-limits