Speaking as a man with the contents of my cabinet shown below, to a degree this is 'do as I say not as I do', but I think the opinion will run true.
With your first gun don't worry too much about a backup.
It's much more important to maintain your gun, understand how it works and be able to diagnose & deal with an issue.
Pistols are fun, and the Tpx (with remote line air mod) is a good choice.
But is expensive to spend on a 'spare' or 'sidearm'
A sidearm for when your main breaks down or is empty of paint is an added weight to carry - you should maintain to prevent it failing & look into possible ways it can fail, if it fails on you then try to sort it in game, then drop out of the game and sort it in the safe zone. If carrying a sidearm for when your hopper is empty then monitor your paint use and learn to load before empty and while firing
However it is convenient and easy (perhaps too easy - you don't learn to maintain) to have a reliable spare. Such a spare should be an 'old reliable', something that you know you can grab out of the bag and know it will work.
A mechanical spare tends to tick a number of boxes, typically because they don't need batteries and are simple - and classicaly these are what people have bought first and learned from the start. But this does not mean you should buy one just to be spare
The Gog Extacy should be reliable, it's essentially a Smart Parts Ion. They are not highly complex and are simple to maintain. The Ion has been the donor to numerous specialist modified builds.
Put together a spares kit - o rings, banjos, internal air hoses, macro line, air fittings and lube. Add a small toolkit starting with either a paintball multitool or screwdrivers, hex key set(s), chrono spanner, o ring pick (or dental pick) and a macro line cutter
Look around YouTube for how to maintain the gun
Plan your future paintball purchases, before buying more guns consider your goggles, air system (more 3000psi cylinders or going to fibre - and the right size for you) and possibly a chrono
There are many threads on such subjects