The following paragraphs are included at Lithium Carbonate - FDA prescribing information, side effects and uses, which is generally a reliable site.

Pharmacokinetics

After oral administration, lithium is reported to be completely absorbed in the upper gastrointestinal tract. Peak serum concentrations (Tmax) occur 0.25 to 3 hours after oral administration of immediate release preparations and 2 to 6 hours after sustained-release preparations.

The distribution space of lithium approximates that of total body water, and the plasma protein binding is negligible. After equilibrium, the apparent volume of distribution is 0.7 to 1 L/kg. Lithium is not metabolized.

Lithium is primarily excreted in urine, proportionally to its serum concentration. Lithium is filtered by the glomerulus, and 80% is reabsorbed by passive diffusion in the proximal tubule. The elimination half-life of lithium is approximately 18 to 36 hours. Lithium excretion in feces is insignificant.

I only ever took Lithium ER, not the instant release. I was always prescribed to take it twice per day, despite it being an extended release form, though I took much higher daily doses than you. In general, I usually need fairly high doses of medications, with just a couple of exceptions.

I hope your new arrangement works out better for you.