Originally Posted by dddd

A few points about your comments above:

Suntour Accushift systems were designed for relatively primitive chains, and as such the shifters are designed so as to "over-shift" when moving to a larger cog. This actually became quite problematic when the 7s versions arrived, and many dealers never ordered another Suntour-equipped bike ever again.

Suntour freewheels, whether used for index or friction shifting, work better when paired with modern, 8s or 9s chain than they ever did back in the day.
The cog spacing on 6s Suntour Accushift freewheels is "standard", same as Shimano 6s. The spacings are uniform, no "uneven spacing". I have used many Accu-6 freewheels together with 9s chain, and shifting always improved, regardless of the shifter used.

I see very few people rebuilding their STI shifters or having them rebuilt. Usually some sort of soak-job is as far as anyone wants to go because Shimano does not supply any small parts for them.
I recommended the Microshift levers because they are new, fairly light, and cheap. I just bought another pair myself.

The 9s chain, especially, will widen the range of rear cogs usable without having to "trim" the front derailer position, so much so that I typically use the entire cog stack together with the big ring over an entire ride.
The newer chains are also far more tolerant of cross-chaining, and tolerant of sparse lubrication, yet can last many thousands of miles.

9s chain sometimes does not play nice with older chainring spacing, but usually it works perfectly under all conditions, even on 40-year-old Schwinns and Peugeots. Certain cranks like older Stronglight and SR Apex have particular problems with even 8s chain though, but this is easily corrected with minor modification to the small chainring, something I have been doing from time to time for over 20 years since it is always worth the effort to be able to use narrower, better modern chain.
Often an 8s chain would be a better choice on an old Accu-6 drivetrain, but note that with Accu-7 the 9s chain improves rear shifting quite dramatically, and Shimano's 9s chains have an edge above all other brands in terms of their ability to grab teeth.

I don't know why your post started with teaching me the merits of using downtube shifters. I do almost all of my riding using downtube friction shifters myself, so am also well-versed in riding with, and fine-tuning these old drivetrains, using modern chain exclusively.

The Sora shifters are a disaster imo, since they require the rider to swing their hand around the shifter in order to reach the mouse-ear lever while riding at high speed. This is many times more dangerous than dropping one's hand down from the drop bar to execute a DT lever shift, since one's hand will tend to bang against and snag the STI lever during that swing-around move, as one is riding over possibly rough ground at the highest speeds (right when you would want to shift to a taller gear). A downtube-lever shift is a comparative non-event! Let me say that for spirited riding at a bike's highest speeds, the Sora levers are dangerous imo, as I have seen riders swerving quite sharply while trying to execute a shift to a taller gear from the drops. It is made only worse when one considers the often-twitchy behavior of so many bikes that have been adjusted to fit the newer, less-fit rider, so I tend to use the mere sight of Sora levers as a warning to stay well clear of the bike/rider I might be passing.

So I will recommend keeping the DT shifters firstly, if the OP isn't set on using integrated shift/brake levers!