You seem to have made a mathematical error and come to some incorrect conclusions. When converting via flawed diamonds:

850 rat shilling buys 400 diamonds with 50 RS remaining. This is sold over 4 actions for 120 Scrip.

Creating a Brass Lollipop takes 4 actions and gives 135 Scrip, but only costs 625 RS leaving an extra 175 RS remaining.

Brass Lollipop is both higher SPA and gives you more Scrip total.

Selling for Feathers we get 106.25 for 850 RS which sells for 106.25 Scrip, not more than 125S as you stated.

Most options for direct conversion from the rat market to Scrip are intentionally bad. It isn't possible for me to evaluate every longer grind, but out of simple grinds Brass Lollipops is simply the best. It may be possible to make better use of RS by buying items you need to fuel some other grind.

The clear answer for a better solution to Brass Lollipops is by making other Brass Skeletons. Brass Lollipops get no zoological mania bonus, so it is better to make a fish, or even a bird if you have relevant torsos. You can turn a Leviathan frame or Prismatic frame into a fish with a brass skull which sells for scrip to an Author of Gothic Tales. You can also make birds with many brass skulls on a skeleton with 7 or 8 necks and sell to A Colourful Phantasist when she appears during bird week. This will give bone market exhaust but it's an extremely good conversion rate.