I went ahead and installed magus. I went five levels into fighter to get extra attack and on level 6 I’m a drop into magus. I have the extended levels mod to go past level 12
if you're going 5 into fighter anyway, may as well just run mystic for diversity. The first 3-5 levels of mystic as a late dip ain't worth the time it'd take to get online.
Especially since Magus technically gets a version of extra attack as it is. The 5 fighter levels are kinda redundant. Green Flame Blade -> extra spellstrike attack > attack -> extra attack from fighter. Does more damage, and does the same number of attacks.
If you're intent on a primary martial... go gloomstalker or hunter ranger. Hunter ranger with colossus slayer will actually give you dramatically more damage /rnd than fighter if you're just going to pivot into a caster. This is not accounting for second wind, but... I usually have a lower regard for that because I have yet to rest more than 12 times across all 3 acts in... 5 runs? If you rest more fighter pulls ahead further. Consistency though, an extra 1d8 on wounded enemies will beat it over time in a walk. Plus potential utility of longstrider and the other rng utility spells.
All this goes out the window if you start talking battle master fighter because of it's debuffing. But I'd say fighter isn't a great base class for a magus pivot unless you're going EK. But like... don't go EK please. It's bad.