Certainly the 24" deep shelf is your best option for larger (deeper) speakers than in bookshelves. Not too deep, some toe-in will be needed.
What height is that shelf? Generally you want your tweeters at seated ear height.
A bit further back than an iso triangle works better for many, so: a few feet from side walls, 18 ft wide room, speakers perhaps 12’ apart, 15’ even18’ back should allow decent l/c/r imaging. against rear wall, probably no real depth imaging.
You could start with your all in one amp, and then some smaller inexpensive speakers intended for a second system later. ’Learn’ the room, mess with locations, heights, toe-ins. After gained knowledge (researching ’real’ speakers all the time), then more informed choices can be tried.
Nothing wrong with a moderately priced set of speakers and better speakers a few years later either. We all have ’moved up’ over the years.