Metta Bhavana is often used in conjunction with Vipassana (Satipatthana) within the Theravada as they are complementary. I am not familiar with the zen practice of shikentaza and nor am I qualified to indicate to you whethr you can practice the two. Perhaps it is a question best put to your teacher or guide.
“No lists of things to be done. The day providential to itself. The hour. There is no later. This is later. All things of grace and beauty such that one holds them to one's heart have a common provenance in pain. Their birth in grief and ashes.”
- Cormac McCarthy, The Road
Learn this from the waters:
in mountain clefts and chasms,
loud gush the streamlets,
but great rivers flow silently.
- Sutta Nipata 3.725Compassionate Hands Foundation
(Buddhist aid in Myanmar) • Buddhist Global Relief