I'm generally surprised they're still using the unmaintained old version of jemalloc instead of a newer allocator like the Bazel-based TCMalloc or mimalloc which have significantly better techniques due to better OS primitives & about a decade or so of R&D behind them.
it seems they have tested it: https://github.com/ClickHouse/ClickHouse/issues/34157
besides jemalloc also being used by other columnar databases it has a lot of control and telemetry built in. I don't closely follow tcmalloc but I'm not sure it focuses on large objects and fragmentation over months/years.
TCMalloc has an absurd amount of bookkeeping and stats, but you have to understand the implementation deeply to make sense of the stats. https://github.com/google/tcmalloc/blob/master/docs/stats.md