@toothandnail :
maybe you should fix your authentication but i think you should have a look at the OpenSSH man page and the OpenSSL man page.
First try on your SME:
ssh -V
on your Slackware :
ssh -V
and on both :
ssh -Q cipher ; ssh -Q cipher-auth
openssl ciphers -v
openssl list-cipher-algorithms
what do you notice ? Do you make a difference between OpenSSL and OpenSSH?

Many ciphers are deactivated by openSSH, to test a cipher, you can try :
ssh $USER@$SERVER -p $PORT -C -c $CIPHER
ssh $USER@$SERVER -p $PORT -C -c blowfish
ssh $USER@$SERVER -p $PORT -C -c chacha20-poly1305@openssh.com
The cipher algorithms are just algorithms for cryptography so now you can read this
page and this
page, to understand what you are trying to do.
A+
PS : french developers should read this
page