OK, I read a couple of the arcs over again, and with all the events of CSV fresh in my head I think there actually is a fairly reasonable timeline that can be made. For some reason I thought Japanese school students had only a one-month break, which isn't true. That was the big stumbling block for me earlier, but now that I know otherwise, it can be wrapped up more neatly. Key to this is that Japan has two separate blocks of school vacation, basically a month in March and a month in August.
In Chapter 1, Minako says that she just started her first year of middle school. Japan's school year starts in April and goes up until the end of July, where they then have a one-month summer break up to the end of August, during which time she goes to Greece in Chapter 7.
Chapter 9 is stated to be shortly before Valentine's Day, so nearly a full year has gone by since the start of Chapter 1. Chapter 10 takes place basically right after Chapter 9, with a scene with Artemis analysing the Rainbow Candy dropped by Ace, and he says there's been little activity going on for quite a while, with Minako not even taking her compact at times.
She would get a brief break from March to early April, after which she comes back to school and then Nyannyan makes her attack. Several chapters go by, up until the Ace arc, which would take place during the August break. Since it's during the summer break, there's no apparent fuss from her parents about her auditioning for a film and going to China. At the start of Chapter 15, Minako mentions that she is in the 'middle' of 7th grade. Starting the school year in April and doing 4 out of 10 months of schooling for the year (April-July) would basically put her halfway through the year.
The only problem is that Minako is stated to be 13 at the start of Code Sailor V, at the start of April, while she's supposed to be 14 when she first shows up in BSSM. This can be solved by the start of BSSM (being in late March) overlapping with after Chapter 10 of Codename Sailor V. This is rather unlikely if Venus is to 'search for the other guardians' at the end of the series, as well as taking on a special agent role with the police department, which gets referenced at the beginning of BSSM Act 1. Not to mention she uses the Game Crown Center hideout in Chapter 15, so overlapping timelines would run her the risk of being discovered by the other girls.
The more likely answer though is that since CSV was the original manga storyline, but wrapped up much later deep into BSSM's run, a few elements will be out of sync. In this case, Minako's age at the start of CSV can just be retconned to be 12 and the start of BSSM will be basically nearly 2 years after CSV, making her 14 when she shows up as Sailor Venus.
On the other hand, Minako states consistently throughout the entire run of the manga that she is 13 years old, even though she must have gone through at least one birthday (October 22nd) so that might just be a non-aging character trope.
so in a nice point form (I have no idea the year BSSM is supposed to start in, I'll just assume 1992):
April 1990 - Minako starts first year of middle school, meets Artemis, becomes Sailor V
August 1990 - Chapter 7, Minako goes to Greece for vacation and fights the Dark Agency there
September 1990-January 1991 - DA activity declines, Sailor V fades from the spotlight during this time
Late January/Early February 1991 - Chapter 9 and 10, Kaitou Ace is first introduced
April 1991 - Minako begins her second year of middle school, she fights Nyannyan and later the other two siblings
August 1991 - Ace arc in China
March 1992 - Start of BSSM