Most of this was likely already fixed by MDEV-27017.
On one implementation of the AMD64 ISA, the test
innodb_fts.concurrent_insert would still occasionally hang,
with both dict_sys_t::evict_table_LRU() and
dict_index_set_merge_threshold() waiting in dict_sys_t::lock()
several threads waiting in dict_sys_t::freeze(),
no thread holding exclusive dict_sys.latch and also no thread
in the stack traces apparently holding any dict_sys.latch,
even though dict_sys.latch_readers == 1.

To prevent this scenario, we will remove the dict_sys.latch
acquisition from dict_index_set_merge_threshold(). It is actually
not needed, because dict_sys.sys_indexes will not change after
InnoDB startup. The SYS_INDEXES leaf page will be sufficiently
protected by the page latch.

There potentially is a bug in the srw_lock implementation,
which will have to be investigated further.