What are the advantages of having more than one bike?
Rule 12 says that you should aim to own N+1 bikes, where N is the number you have now.  While that's completely tongue-in-cheek, there's some merit to having gear fit for purpose.

For example I have a fast road bike, which is uncomfortable in the rain and useless for carrying stuff.  I also have a 20" folder with carriers and mudguards/fenders, which is okay for moving things, towing a trailer, and works well in the rain.  But fast it is not.

The advantages of multiple bikes is flexibility


Each can be set up for specific types of riding
You can have a dedicated rain bike without having to add stuff
Spare bike to ride if one is down for some reason
Spare bike to offer to other people if they visit


The disadvantages are


Extra cost - bikes aren't as expensive as cars, but aren't generally free either.
Storage space - you can only ride one at a time, the rest will be stored somewhere.
Upkeep - more bikes means more things to maintain and service, but your mileage is spread over more things.
Records - you will need to keep better records over what was changed when, and how much mileage a bike has done.  I use strava for this.




In your case, the bike might be "over the recommended load" but you're working on that BY riding.

If you find that the bike is not performing to your needs then an additional/replacement bike might help.  This is different from you not performing as expected!

What would a fatbike do for you that your current bike doesn't?  It would likely go slower, weigh more, be harder to climb grades, and tubes/tyres are expensive.  On the plus side it would "float" over sand snow and loose dirt better, but it is not necessarily going to be any stronger or tougher than your existing bike.

You're only 20 kilos heavier than me, and you've already uprated the rear wheel.  I'd suggest riding it while you work on your fitness, and plan on a replacement bike in a couple years.