Delete arm 14 to make it loop. I assume many other solutions will have a similar motif.

I had many almost-solutions that consumed input at 4R (see showcase submission for one) but eventually gave up and settled on 8R. At this slower rate, each extra monomer costs 16 cycles, so it is important that the monomer is bonded to the polymer as close to the output as possible. Therefore I wanted a monomer that contained all the triplex bonds. Since one of the horizontal bonds must be in the monomer, that leaves only two candidates for 6 atom monomers, of which I found this one to be more reasonable.

I think the 3x2 monomer (fire-salt-fire parallel to fire-salt-fire) is easier to build and work with but that requires doing a triplex bond in the polymer and is likely to be slower when consuming input at 8R.

I was able to get surprisingly far at period 3 but it seems to require height 5. But would Zorflax make a week 6 HPC puzzle with a (relatively) easily achievable min at 4/4?