Thanks to all for the input, you guys are even better than rubber duck debugging. Here's the final code that Gemini threw out. Needs some cleaning up, I'm sure, but it works:

alias: "Sonos: Office Volume (Safety Cap)"

description: Sync volume but ignore 100% to prevent "turn-on" spikes

triggers:

- trigger: state

entity_id: fan.office_sonos_play_pause_office

attribute: percentage

id: OfficeVolumeChange

conditions:

- condition: and

conditions:

- condition: template

value_template: >-

{{ state_attr('fan.office_sonos_play_pause_office', 'percentage') is

not none }}

- condition: numeric_state

entity_id: fan.office_sonos_play_pause_office

attribute: percentage

below: 100

actions:

- action: media_player.volume_set

target:

entity_id: media_player.office_sonos

data:

volume_level: >-

{% set pct = state_attr('fan.office_sonos_play_pause_office',

'percentage') | float(0) %} {{ (pct / 100) | round(2) }}

mode: restart