index maintenance strategy if few insertion points relative to # of rows
A page split is a page split.  You can't control what SQL Server does when it requires a page split.  That is not a variable and configurable setting.  When SQL Server needs to split a page, it'll put roughly half of the data in one page and half in the new page.

If you are having "multiple" insertion points into the index, causing hot spots in the index to undergo many page splits and much fragmentation then there isn't much you can do to alleviate that with the current design.  It sounds like the way your data is dispersed that could be the root of your problem.  If you insert into the middle of an index, you will cause page splits/fragmentation.

There are a couple of things you can do, depending on how predictable your "insertion points" are.  You could utilize table partitioning, and this would allow you to rebuild the indexes on a particular partition (or partitions).  This will keep cold data from receiving the maintenance, and minimize the duration of the rebuild to the hot and fragmented partitions.

Depending on your edition, you could also utilize online index rebuild operations to reduce the availability impact on this maintenance procedure.