Could you share with us, how you solved your issue?
Sure. This is what I do now in the tag mappings:
DISCOGS_MAIN_RELEASE_DATE <- $if($or($strcmp(%MASTER_RELEASE_MAIN_RELEASE_DATE_RAW%,%RELEASE_DATE_RAW%),$strcmp(%MASTER_RELEASE_MAIN_RELEASE_DATE_RAW%,$left(%RELEASE_DATE_RAW%,4))),,%MASTER_RELEASE_MAIN_RELEASE_DATE_RAW%)
DISCOGS_RELEASE_DATE <- %RELEASE_DATE_RAW%
And I use it in SimPlaylist's Sorting pattern like:
$if2(%discogs_main_release_date%,$if2(%discogs_release_date%,$if(%date%,%date%'-00-00','0000-00-00')))+$if2(%discogs_release_date%,$if(%date%,%date%'-00-00','0000-00-00'))+%path_sort%+%tracknumber%
And in SimPlaylist Columns (just a single column) like:
[%discogs_catalog%|]
[%discogs_label%|]
[%discogs_country%|]
[Main: %discogs_main_release_date%|]
$if(%discogs_main_release_date%,[This: $if2(%discogs_release_date%,%date%)|],[$if2(%discogs_release_date%,%date%)|])
[%discogs_release_credits%]
I removed some 'compatibility' with previous tagging habits. I hope I didn't mess things up. The sorting has a flaw when the main release date is not like yyyy-mm-dd, but I didn't feel the need to fix that (yet).
If it doesn't explains itself, let me know.