Firstly, I would indeed suggest that you make the changes you yourself proposed, ie Delta Gardevoir and Delta Haxorus for Magnezone and Feraligatr. Delta Ralts can be found in a house on Route 4 and Delta Axew is found by doing the trade outpost quest in Miara Town, Feraligatr is fine to use up to that point if you want.
In terms of a sixth team member, I would say that Blaziken is a good choice. Fire and Fighting are types your team doesn’t have and are generally good to have, and Speed Boost + Swords Dance makes Blaziken pretty ridiculous in my opinion. Downside is that the earliest opportunity to find it is Combusken with 6 badges in the second slot of the friend safari (see the wiki page to find the particular online safari you need for it).
This isn’t super late but its late enough to miss quite a bit at least, so in the meantime there are some other fire or fighting types you could use, like Lucario (Riolu can be found in the ruins on Route 2 after the minor events there), Delta Scizor (Delta Scyther can be traded for on Route 3 for a steel type, like Magnemite from Cyan Cavern for example. Evolution is honestly optional with this one since Scyther with Hustle is faster and stronger than Scizor lul. If you do evolve it though, the Delta Scizorite is on Route 6), Charizard (Charmander can appear in Route 3’s hidden grotto), or Typhlosion (Cyndaquil appears on Route 4, added bonus is the Typhlosionite can be gotten in Sonata City, still a little while away from getting Blaziken).
Edit: Since you don’t have a flying type either, you may also like to consider Fighting/Flying type Delta Bisharp. Delta Pawniard can be found at the Power Farm in Nasca Town. Like Blaziken this is somewhat late, although not quite as late (a couple of routes and towns earlier). Hidden Ability Wind Force is fantastic (flying-type motor drive), and the mega is pretty silly as well, having technician power-up punch as well as 155 attack and 120 speed and a pretty good movepool.
I’d say on balance Blaziken is stronger, although the Blazikenite isn’t until Victory Road, whereas the Delta Bisharpite is at the Power Farm using Tesseract (which you can use after the Dragon Ruins events), and Mega Delta Bisharp is very strong. Either way, I think both of them are very good options to use on your team.