DOUBLE is SMALLER (good), but DECIMAL is precise (more imporant), so go with DECIMAL.
A PRIMARY KEY is a "UNIQUE KEY". So the "partition key" must be added (preferably at the end) of the PK (if it is not already in the PK [as is your case]). It's a rule; live with it.
A lot more rules, tips, etc:
http://mysql.rjweb.org/doc.php/ricksrots
A Summary Table would need to be updated as new information goes into the main table. There are many ways to do this. An immediate UPDATE is probably the most costly. I prefer a periodic "INSERT INTO summary SELECT ... FROM main GROUP BY...". This is very efficient; but it may or may not work in a given case. By "periodic", I mean every hour, every minute, and/or on demand -- again this varies with the use case.
Sorry, I don't have a document on Summary Tables -- it is a difficult one to write because of all the variations.
I have read 27 of your questions on this set of forums, and have responded to most of them. This thread is a significant escalation of your requirements; perhaps should hire an experienced DBA.