If the Rams cut Boom they will have $15M in pro-rated bonus money to account for, $5M in '24, '25, and '26. They must take a cap hit for all of that, but they would have options, as you point out, as to how they want to absorb that hit.
If they cut him March 17 without designating it a post-June 1 transaction then all $15M comes due at that point and is applied to the 2024 cap.
They could cut him March 17 and designate it a post-June 1 transaction. Doing that will cause the 2024 pro-rated bonus ($5M) to go against the 2024 cap, showing a cap savings in 2024 of $15M.
But then the '25 & '26 bonuses will both be applied to the 2025 cap resulting in $10M in dead money.
The Rams will only realize a net of $5M in cap savings either way, but I can see how they might want to defer some of the pain to 2025 in order to maximize their spending power in 2024.
AlbaNY_Ram