Will it actually charge faster if I buy a 2.4A wall adapter, or is it already a limitation of a 5W charger itself that it wouldn’t allow “faster” charging? To me “faster” charging is already if I plug my phone into iPad’s wall charger, for example.
No, a ~2A adapter is the minimum needed to charge at 5W, because a 1A adapter alone isn't sufficient, given the aforementioned inefficiency and losses.
(Plus, for whatever strange reason, the cheaper Anker wireless chargers being sold now don't support Apple's 7.5W profile in any case, regardless of the size of the adapter).
7.5/10/15W also need headroom, so for those levels, a QC 2.0 or higher adapter is needed, as they can supply up to 36W at various voltage/current levels. PD can theoretically do the same (and higher), but QC has been the preferred standard, given its prevalence and corresponding cost advantage.
QC 4+ will support PD protocols up to 27W, which should simplify things, and it may ultimately not matter whether a user plugs future chargers into a QC4+ or PD adapter, or which protocol is actually used.
Ultimately, it comes down to getting sufficient input to produce a specific output. That can be achieved using common USB-based industry standards like QC and PD, but also with a proprietary non-USB adapter, like some of the more expensive multiple-coil "base station" type chargers from Belkin, Nomad, etc. employ.
