Tires and rims for touring- different in front and back?

I ride a aluminium frame trekking bike close to its maximum weight capacity of 136 kg. With extra food and water for wildcamping I guess I'm often 5 kg above. I've toured 12.000km like this but always felt a little uneasy on bad roads and my next tour will take me through Spain for 3 months and I might have to increase the load even more.

I just ordered a new l backwheel with a wider rim that can carry up to 180 kg.

My reasoning behind that is, that this might be the weakest link and I dont want to invest to much money into the bike for the front wheel, because sooner or later I want to get a real steel touring bike and that's were I want to invest.

I run 42mm Schwalbe Marathon at the moment.

I have the following questions:

1. is there any downside to having a wider rim on the back than on the front?

   1. I think I could fit 44mm tires as well. Is that 2mm difference recognizable in your experience?
   2. I hear mountainbikers run bigger tires in front for control, but never read about touring bikes with bigger tires in the back. I had the idea it could help on sand or gravel to prevent sinking in and slipping, since yhe back carries more wheight. What do you guys think about that?

Any advice and shared experience is appreciated. Thanks!