Great stuff Don! I agree this word appears to have been important for Epicurus, but I cannot add much to what you have said.
"The word ataraxy implies a metaphor derived from the sea and the weather. One of the original synonyms is “calm,” galenismos, of which the proper application is to the sea, tranquillitas in Latin." (DeWitt, pg 225)
For Galenismos:
ὁ γαληνισμός ‘tranquility’ ‘peace’ or ‘calming effect.'
'Pacification' Oxford Handbook (pg 442)
From ἡ γαλήνη ‘stillness of the sea’ ‘absence of wind’ ‘calm,’ ‘calmness of the mind’ ‘serenity’
The famous doctor Galen, born about 450 years after Epicurus, also got his name from this word.
"...and as for all who are not fully among those on the way to being perfected, some of them can from this summary obtain a hasty view of the most important matters without oral instruction so as to secure peace of mind." (83b, Bailey)
"...while those, on the other hand, who are not altogether entitled to rank as mature students can in silent fashion and as quick as thought run over the doctrines most important for their peace of mind." (Hicks)
"...will be able in their minds to run over the main of the essential notions, and to derive assistance from them for the tranquility and happiness of life." (Yonge)
A bit more about Engalenizein:
"I who urge upon others the constant occupation in the investigation of nature, and find my own peace chiefly in a life so occupied, have composed for you another epitome on these lines, summing up the first principles of the whole doctrine." (37a, Bailey)
"I, who devote to the subject my continuous energy and reap the calm enjoyment of a life like this, have prepared for you just such an epitome and manual of the doctrines as a whole." (Hicks)
"I recommend them, while still pursuing without intermission the study of nature, which contributes more than anything else to the tranquility and happiness of life, to make a concise statement, or summary of their opinions." (Yonge)