I'm assuming whatever spending advantadge Dems had was more or less squandered the moment those billionaires got cold feet and decided to oust Joe Biden instead

I'm assuming whatever spending advantadge Dems had was more or less squandered the moment those billionaires got cold feet and decided to oust Joe Biden instead of relying on the incumbent vote. That disruption very likely scared off several donors. But yes, there was also simply a lack of energy in the D voter base in 2024. No amount of spending can recover disengaged voters who simply dismissed everyone as "equally bad".