For Shimano products:
Your FC-R8000 crankset is 110 BCD 4 arm. The minimum size physically possible for 110 BCD is 33T.
https://si.shimano.com/#/en/EV/FC-R8000
https://specialites-ta.com/double/10...denture,33-dts
If you want the FC-RX600-11 or -10, it is 110/80 BCD 4 arm. The minimum possible size for 80mm is 26T. I was wrong when I said 80mm was minimum 28T. TA makes rings for 80 BCD 4 arm down to 26T, label says 10 speed but the webpage says 11 or 10 speed. But they don't seem to make 80mm 5 arm. Both FC-RX600 and the FC-R8000 use the standard Shimano road BB with 24mm spindle, so you can simply reuse your old BB. However, the bolts thread directly into the flange, so there is no way to add a bashguard.
https://specialites-ta.com/accueil/3...denture,26-dts
If you want smaller chainrings down to a minimum of 24T, you need 74 BCD. FC-5703 and FC-4703 are 130/74 BCD 5 arm. You can use the inner 2 and put a bashguard on the outer 3rd. The only problem is that your chainline will be around 2-2.5mm further inward. You could push it out by 1mm if you use the mountain BB cups, with the road water sleeve (but you have to shave off the plastic stopper edge). The Mountain BB cups are each 1mm narrower than the Road BB cups (on each side). So your spindle will get an extra 2mm of length. Then you add 2mm of spindle shims to the right side crank (or 2mm shims to the right side BB cup, which shifts your chainline by 1mm to the right. You could possibly get away with adding an extra 0.5mm of shims.
https://productinfo.shimano.com/#/sp...t%20Chainwheel
Otherwise, I use the Sugino OX cranks, which are 110/74 BCD 5 arm double, which has a standard road chainline. It also uses a 24mm spindle, and the Sugino BB is somewhere in between the widths of the Shimano mountain and road BB, about 1.6-1.8mm narrower overall than Shimano road BB. So I used the Shimano mountain BB with shims. I was also able to add a bashguard to the outside of the Sugino crank but I had to make custom spacers for it. The large ring is in the "middle position" since it is mounted on the inside face of the flange. I've only found maximum 46T middle position from Spa Cycles, TA, and Sugino.
https://www.suginoltd.co.jp/us/products/road/ox.html
Otherwise, you could go with square taper JIS BB and use any cheap 110/74 triple crank.
If you want to go even smaller, the Shimano mountain cranks are usually 96/64 BCD 4 arm. 64 BCD goes down to 22T. However, I've only seen up to 38T for the 96 BCD 4 arm middle position, and TA doesn't make any for 96, only 64, 104, 110 BCD. You could laser cut an outer bashguard that serves as an adapter between 96 and 104 BCD (or even 110 4-arm) by rotating the holes by 30 to 90 degrees. Then you can use TA 104 BCD middle position up to 42T, or TA 110 4-arm outer-"middle" from 42T through 56T. (You could even make an adapter for 5-arm) The Q-factor of the Shimano FC-M9000 or FC-M9020 are the narrowest of their mountain lineup. Chainline is at 48mm, but you can space it inward, but you'll have to use your own longer bolts, like from Truvativ. Otherwise, there are older Shimano mountain cranks with 104/64 BCD, but the Q-factor is wider.
They use the Shimano Mountain BB, which is 1mm narrower on each side (total 2mm) than the Shimano Road BB, so you will have to change your BB.
https://specialites-ta.com/triple/38...denture,32-dts
https://specialites-ta.com/double/11...denture,52-dts