[Jeff721]

@Jeff721 Then this should work for you. Again - PLEASE DO NOT USE UNLESS YOU ARE AN EXPERIENCED USER AND/OR ARE FULLY AWARE OF THE RISKS!

This assumes some knowledge of being able to use SSH to navigate and issue commands and the ability to use the vi editor (see note below).
First you need to enable SSH on your NAS. I highly recommend that you normally keep SSH disabled and once done with this procedure you disable it again. Also you will need to use something like PuTTY to SSH into your NAS.

To create my.cnf file - 

Logon using SSH (need to login with user/pw that has admin)
switch to root - sudo -i
switch to correct directory - cd /var/packages/MariaDB10/etc
List files - ls
if my.cnf does not exist create it - touch my.cnf
edit my.cnf - vi my.cnf

add the following - 
[mysqld]
optimizer_search_depth=0

switch out of edit mode - hit esc
save and exit - :mq

restart MariaDB - /usr/syno/bin/synopkg restart MariaDB10

exit SSH - exit (may have to enter exit multiple times until PuTTy screen closes)

Note - see vi editor cheat sheet here.

Thank you so much @wags1 ...  I was able to permanently fix the variable issue in MariaDB 10 for once and for all!   Thank you for your help...  Much appreciated!

[Jeff721]

One other small comment, if you're not used to or comfortable with vi as the editor, you can install nano on the Synology through the SynoCli File Tools package from the community section in the package manager (from Synocommunity).

Just in case it's useful, as I'm a Syno owner myself who doesn't like vi so much (too much time with Pi's and nano ).

Thanks for the tip @DarrenHill !  I will have to look how to install packages from the SynoCli community as, like you said, working wit Vi is quite challenging!  Nano will be much easier for someone like me who starts in this "advanced" world of IT.   Thanks again!

[wags1]

@Jeff721 YW. Glad you were able to get it done.