How to split PMAX for a table partitioned by RANGE subpartitioned by HASH

So I have a table partitioned by range on a date column and then subpartitioned by hash.

Each range partition represents a year but I only have three years (2002, 2003, 2004) which means PMAX has a lot of rows (140 million and change).

Then each yearly partition is subpartitioned into 8 hash partitions yielding a total of 32 subpartitions going into 8 different tablespaces.

How can I split PMAX while consolidating all the HASH partitions into a single tablespace?

Im on 19.10 so I want to use the ONLINE feature.

Thanks

Roberto.