I'm not sure how experienced you are with commands, but if you need me to join your world to help you develop a spectator mode I will. My gamertag is: @Silentism. For now I will give you a brief explanation of how I would go about creating it.
Spectator mode is possible in bedrock edition. Its a little tricky since commands can't change permissions. I have never created a spectator mode myself, but off the top of my head heres how it would be executed.
1)Create a tag:
5)Remove ability to build/mine
Result: Now you have an invisible player that can't receive damage, can't deal damage, can't build/mine, and can fly around and spectate. The biggest flaw is that they will still be able to access chests and use buttons/levers. When the player leaves "spectator mode" you will need more commands to clear their effects and change there gamemode and /mayfly settings back to normal.
I hoped this helped. Creating something like this is very time consuming and uses a long list of commands. This could be considered a very complex and high-end creation. The key is to keep your commands neat and organized so they can easily be adjusted when necessary. If I join your world and do it for you, i'd ask that you credit me along with my realm code in return.