Makes sense.
If I had a choice of only one one I would get a cordless Combi drill but if you can afford it then the twin pack is the way to go for decking.
The impact driver is the right tool for decking screws, but you’ll likely want a drill on hand as well. I certainly used both when I did my decking.
An impact driver is far more capable at driving screws into wood and getting them below the surface. Just make sure you buy proper impact driver bits which match the screws.
I’d buy the twin pack I linked above, there are cheaper twin packs but they are brushed tools which are bigger and not as powerful. The brushless versions of the tools are more compact, easier to use and batteries last longer.
The 2.0ah batteries are fine for DIY use IMO, if you are a trader, work quickly and need to guarantee all day battery then sure spend more and get bigger batteries. They do add a bit of heft to the drill too.
The only place where getting the smaller batteries fall down are if you want to get into cordless circular saws and angle grinders etc. Those sorts of tools will chew through a 2ah pack very quickly.