The composition of the biosphere is a fundamental question in biology - how much do we have of anything?
Yet for most of the history of biology, no global estimation of the biomass of organisms existed. Only in 2018, a paper was published that estimated the global biomass of different taxa (groups of organisms) . The results are nicely summed up in figure 1 in the paper.
I am convinced a similar project should be done for the neural mass, and capability for well-being, on Earth. My guess is that well-being capability should be explored through intelligence, as the capability for well-being is probably highly correlated to information processing, i.e. intelligence. We know that neuro-mass is not highly correlated with intelligence  (think of a human and an elephant), and thus I will guess it is not highly correlated with well-being capability. On the contrary, there are other measurable factors, such as cortical neurons and neuron packing density, that are known to affect general information processing capacity , and are known for at least some animals.
This project will be a milestone in cause prioritization & resource allocation, as it will allow us to understand which groups of creatures have the largest capability for suffering and joy on Earth. For example, think of the implication of a finding that non-human organisms have a global 10x well-being capability, or vice versa.
I myself only have high-school education and little free time, so unfortunately I don’t think I’ll be able to provide a lot of help with the project. But I’m sure it could be done!
Also, I suppose this idea could be a decent idea for scientific paper.
1 - Bar-On, Y. M., Phillips, R., & Milo, R. (2018). The biomass distribution on Earth. Proceedings of the National Academy of Sciences, 115(25), 6506–6511. https://doi.org/10.1073/pnas.1711842115
2 - Dicke, U., & Roth, G. (2016). Neuronal factors determining high intelligence. Philosophical Transactions of the Royal Society B: Biological Sciences, 371(1685), 20150180. https://doi.org/10.1098/rstb.2015.0180
FYI, the folks at Invincible Wellbeing are working on essentially this question. Some of their draft research is available for viewing, but I don't think they have anything fully published yet.
Thanks, I see it's very hard to think of something that hasn't been already thought of by EAs. By coincidence, I've just seen a post by 'Rethink Priorities' on the subject.
FYI, I have done a simple BOTEC for something similar to what you are suggesting here (see 1st table).