Seat packs are the modern alternative to racks and panniers - but some do feature a lightweight supportive rack. These offer great versatility as you can fit them to almost any bike in a matter of minutes (you'll need to look at specific models if you run a dropper post though).
These seat post bags typically mount using straps around the seatpost and through the saddle rails. Check that you have plenty of seatpost available for the size of the bag attachment, and smaller riders may find they'll have to opt for a smaller capacity bag here.
Waterproofing varies between models and also has a noticeable impact on cost. Opting for a fully waterproof seat pack (or any bikepacking bag for that matter) will make your life much easier at the end of a long, wet day in the saddle. Alternatively, invest in some good dry bags to pack inside your bags to keep your kit dry.
Often with a really generous capacity, a seat pack can be a great place to store bulkier items such as your sleeping bag, clothing and bivvy bag. It may take some time to get used to the feeling of riding with a full seat pack, as your centre of gravity will be raised higher which will affect handling.