Great questions and I'm hoping to say just enough without saying too much here. The early stages of the process are the same for all students, serving as a key foundation, and then, as one progresses, there is the possibility of differing paths. Sādhanā at first definitely entails mantra japa and visualization. (But, speaking from my own experience, mantra japa in Vimarsha is very different from what I imagined it would be, as it's not just about focusing on recitation or concentrating on a sound, but there are really important oral instructions on mantra practice that make it a complete path in and of itself, such that the first initiation can take one completely to liberation).

Something that's interesting about the tantric practices in Vimarsha and Śaivism overall is that these formless experiences are part and parcel of larger sequences. After all, the goal in Śaivism isn't to just live our lives in a formless thoughtfree state, but to be able to enter and exit the various states, moving freely. So, as the process proceeds, you'll find that there are indeed profound instructions for what one might think of as formless meditation, but also that these instructions are integrated into broader sequences of practice such that we see the full dance between forms and formlessness, and learn that the forms and the formless are all manifestations of and in that one Paraṃaśiva. Hope this is helpful!