Photon gets frozen after some time running

[ScruffR]

I'd start with getting rid of all these String() statements and rather use snprintf().
Additionally I'd consider wrapping your readings up into a single event (e.g. as JSON string).
I'd also separate data acquisition from data transfer.

You may also want to not unconditionally call client.connect() when you potentially still have a (re)usable connection.

Which brings us back to some discussion we already had a while ago

@ScruffR

I’d also unify the publishes (MQTT or Particle) into one single string to reduce overhead.

BTW, it would make somewhat more sense if you had one function that does all the publishing, and two independent functions that do the reading of the individual sensors.

With splitting your logic up into dedicated functions you want to reduce redundancy, but with your functions you have both deal with the MQTT stuff and when you need to change anything on that logic (i.e. add the reconnect logic when client.isConnected == false ) you’d do that twice for the sake of consistency. With only one publish function you’d have it both: minimal effort but consistent.

With each instance when you were given some hints and then not applying them, the urge to help keeps getting less and less.
In that very thread I even provided a fully working code that does all that and most likely would not suffer from the problem you are facing now.