74 BCD can only go as low as 24T.
I believe the smallest 64 BCD big brand chainring is 22T. But you can buy these 20T chainrings on Amazon and eBay, but you might have to shave a little bit of the edges of your crank flange to clear the chain.
https://www.amazon.com/aMTBer-20T-Ch.../dp/B00CDW9CC0
https://www.ebay.com/itm/20T-or-21T-...D/332668667006
Some reviewers wrote that they run 20/32/44 triple.
Every 2 teeth of change will affect the diameter by 8mm. 2 teeth equals 1 inch or 25.4mm. So 25.4 / 3.1415 = around 8mm (8.085mm) of diameter. So for example, if you change from 48 to 44T, then you are lowering your FD by 8mm.
I have a 12-34T 10 speed cassette which I mixed myself. You have to be careful, as some Shimano cassettes don't match well with each other. Some cog groups don't work at all together. If you look at the exploded "EV" drawings, you'll be able to find out which groups come with a 12T cog that have a serrated surface to take a lockring. Usually, you find them by comparing the part number. Some groups with 11T first position come with a 12T that can work in 1st position. You simply drop the 11T, add a new cog somewhere in the middle, and replace the 11T lockring with a 12T lockring. If you do use 12T first position, you must also use the specific 13T cog that comes with it, in order to get the correct spacing with the built-in spacer.
Alternatively, you can get a custom mix of Miche on order. I bought 2 to 3 Shimano cassettes to mix mine. But for the price of 3, you can custom order the Miche.
https://www.miche.it/en/products/str...e-pignoni.html
https://www.miche.it/pub/media/produ...t__primato.pdf
I found this shop on Google that assembles custom 11 speed miche cassettes. Maybe you could ask them for a 9 or 10 speed. (Edit: 9 speed only goes up to 29T. 10 speed only goes up to 30T. 11 speed goes up to 34T)
https://thecycleclinic.co.uk/product...r-shimano-sram
On an old bike, I had a 9 speed 12-34T. I don't remember how I mixed it. But I think I took the Shimano HG400-9 11-34, HG50-9 11-30, mixed them, dropped the 11, and bought a spare 12T lockring. But I really don't remember. You can look at the exploded diagrams for a better solution.
(EDIT: I looked at the interlocracing site, and the 9speed 12-34 12/13/14/16/18/21/24/28/34 looks very similar to what I had. Alternatively you could use your current 11-32, drop the first 2 positions, and replace them with a pair of 12-13 from a Shimano group that has the serrated surface and a 12T lockring. There are some websites that sell each cog for around $5-6. but be careful, as I had problems with mixing a 10 speed set where it wouldn't shift between 14 15 and 16 when I tried to insert a 15 from a different group)
On my current bike, I have 12-34T 10 speed cassette, and a 24/44T double chainrings. There is only a few millimeters of clearance between the chainstay and the FD cage and chain, but I have a 20" folding bike, so you might have a little bit less clearance if you have larger wheels.
