As with most hobbies, once you have bought the main equipment there is then a world of other things that can be purchased. Some are very useful and will help and improve your shooting while other stuff is great for presents or ‘just because’.
It’s easy to get lured into extensive modifications and additions but for the best part rifles work better without them. Adding things to the end of the barrel like splitters or breaks can make an improvement in group size but often it’s purely down to adding weight in the right place that reduces rifle movement during the shot cycle rather than any specific characteristic of the modification. A few bits of wheel balancing lead will often achieve the same result. For target shooting butt hooks are a useful (although expensive) addition but they need to be part of a well-fitting stock to really deliver benefit.
Silencers are best avoided for target shooting, they can cause accuracy problems and even a really loud muzzle crack is unlikely to startle a metal target!
However, an important thing to consider is your choice of pellets. Most rifles will shoot best with a particular type of pellet and this can really only be found out by trial and error but JSB exact 8.44 grain in 4.52mm is usually a good starting point. Pointed hunting pellets are best avoided as are novelty pellets (unusual shapes or plastic coverings), they just don’t work consistently well at the ranges we shoot.
Apart from the main equipment needed for the sport, it is really up to you how much you buy. Some things, like a chronograph and barrel cleaning kit, are things you should consider getting fairly early on and PCP rifles need something to fill them with, either a dive tank or a special pump.