As I’ve been reflecting on this more, I realize that these 5 practices differ also in the mental strategy that the Buddha outlined. Specifically, the mindfulness (MN118) and recollection (AN11.12) practices involve an immersion of our mind into the meditation object, such as our breath, so that we become fully aware of that meditation object (some like to use the word “concentration”, but if we look at the Buddha’s metaphors, such as a lotus “steeped in cool water”, I think immersion makes more sense).
For the brahmaviharas, though, a different type of mental activity is occurring. Specifically, we are spreading thoughts of goodwill, compassion, sympathetic joy and tranquility to all beings in the six directions (north, east, etc.). We are not immersing our mind, but instead, spreading our positive energy.
Lastly, in the emptiness practices (MN121), our mental activity involves “stepping back” from things, until they become infinitesimally small.
Ultimately, all three of these general mental approaches (immersion (MN118), spreading (AN10.208), or stepping back (MN121)) lead to an expansion of the mind and a progressive diminishing of our sense of self to the point where we can fully understand anatta, anicca and dhukkha.
These three general approaches serve as the vitaka and vicara (directed and sustained thought) that get us to the first jhana. These three general approaches create a sense of seclusion/security/peace which creates the joy/happiness necessary for the jhanas (the five elements of the first jhana are directed thought, sustaining thought, joy, happiness and one-pointedness). We can then stop directing effort at moving our mind to the meditation object (vitaka and vicara are not part of the second jhana) and move onto the higher jhanas. Of note, if our practice involves a meditation object that involves physically moving our body, such as mindfulness of postures (such as mindfulness of walking), we may not be able to enter a jhana state because we need external situational awareness. But in all the other cases, we may be able to progress into the deeper jhana states.
The Buddha has provided us with a relatively straightforward roadmap. It is fascinating how, in the subsequent centuries, we have complicated it so much and become so confused, with all these different teachers (writing 100s of pages of instructions) and approaches. The reality is that the Buddha’s straightforward meditative process tends to unfold naturally as we practice it; in some ways, learning too many details actually starts to get in the way because it creates excessive/extraneous thought that actually distracts us from immersion, etc.