There are several things that I would try. We must start from the base that the export is interrupted by exhaustion of the maximum time (time_limit) or memory.
First, see if you can download by compressing with zip or gzip:
Second, see if increasing the time limit to a very loose time changes the number of rows exported. In
$cfg['ExecTimeLimit'] = 3600;
I do not think the export lasts an hour ...
Third, increase the memory limit. Also in
$cfg['MemoryLimit'] = '512M';
Fourth, try to save the file locally on the server side instead of sending it to your browser. In the same
$cfg['SaveDir'] = __DIR__;
That will save the CSV at the root of your PHPMyAdmin installation.