I ask in addition to existing DefaultDbCachePages add new firebird.conf/database.conf parameter
DefaultDBCacheSize which can be specified in kilobytes, megabytes or gigabytes of memory directly.

For example,
DefaultDBCacheSize = 8m
i.e allocate 8 megabytes of cache for the DB.

Of course, engine can divide this value by page_size and select closest round number of pages for cache.

If both DefaultDbCachePages and DefaultDBCacheSize specified, bigger value can be taken.

As alternative to gfix -buffers n, I think, need to be added new parameter. maybe -cachesize n ?
Also we need new dpb parameter like isc_dpb_num_buffers.