(I could swear this issue is registered already but I did not find it, so I opened a new ticket 😂)

ClickHouse should have logic which converts low cardinality data to type LowCardinality. This could either be done on the level of parts (where it may create consistency issues with the type as defined via DDL) or by a mutation that converts the column to the LowCardinality type in the background.

Kind of related: #70605

Cardinality statistics (ticket) will be very helpful to control the conversion.