I'd also suggest practicing nadi shodhana pranayama with breath retention. There is some evidence that holding your breath in a controlled way exercises your spleen, like endurance divers train for, which injects oxygenated red blood cells into circulation. [1][2] It obviously has an effect on respiratory capacity as well. [3]

There are plenty of resources on Nadi Shodhana -- I'd suggest Iyengar's "Light on Pranayama" -- but I'd recommend starting with a 1:1:1 ratio of inhale:hold:exhale counts through alternate nostrils, then moving on to 1:2:2 and then 1:4:2.

[1] https://vcresearch.berkeley.edu/news/enlarged-spleen-key-div...

[2] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3694106/

[3] PDF warning: https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/EP086...