Setting up a persistent roomscale stage

I feel a bit confused about aligning the scene with the real world play area.

The play area is an empty 8x4m room dedicated to VR games. The player will be prepared and equiped in a small room next to the play area. When ready, he walks to the play room in passthrough vision mode, he takes place to the starting location of the play area, when done the game starts. 

Questions:

- The floor in the play area is a perfect rectangle 8x4m, should I use Scene API (and how) to define perfect planar bounds instead of the manual Guardian setup with controller? (is Scene API only relevant for Mixed Reality or is it also suitable for full VR?)

- While actually testing my game with manual guardian setup, the room occasionally shifts or rotates in a wrong axis, etc. (pretty scary because I don't want to have to setup the room again while players are waiting to play their game) So how can I ensure a persistent play area? Does Spatial Anchors may be useful?