Greetings, all!

I heard the best time to multiclass out of a base caster class (Mage, Bishop, etc.) was after level 24. This was due to being able to cast all spells from that class at maximum power level with no spell fizzling/miscasting assuming 100+ skill in the spell school (Wizardry, Divinity, etc.) This also assumes I have all the spell picks I want for this class, which is likely true.

Hybrids are 4 class levels behind full casters. For spell purposes (though not necessarily for other purposes), they cap at level 28.

Thus, on this basis, is it wise to swap character classes after maxing casting? If so, to what, and in what contexts?

Thankee!

It depends a bit wich wizardry you're playing.
Wizardry 7 had easy exp. compared to some of the others and is quite long making mulitclassing always a good choice, W8 had quite a grindy start if you started a new game without importing a party, W6 was short compared to 7&8 and you got more skillpoints etc...

Aside from the this...
Starting with W7 or maybe even W6, can't remember, you keep your skills after changing the class, even if the new class did not have the skill, for the older games you keep only the skills of the class/race, losing skills if you changed to basic class from a hybrid or prestige class...so switching class again after playing a hybrid/prestige class was a bad idea for the older games but for W7 it was sometimes good to switch to class X for some level just to get a special skill.

Each time you change classes, the stats change to the higher values of Racemin./new classmin, if you changed to a hybrid or a prestigeclass and then change back you will loose statpoints, so switching again after maxing a hybrid/prestige class you might loose quite a number of statpoints.

Equipment, most wizardrys had some epic equipment for special race/class combos, if you got a set of those..changing could be good just to be able to use that stuff.

Party composition...don't forget your playing a party, it's always important to have a good composition, so sometimes changeing the class could be good even if the charakter looses some strenght but would strenghten the party as a whole.

Have fun ;)

Talking about W8, with a 6-8 people party you will finish the game around L20. In these games you won't reach L24/L28.

There is no reason to change into sth else after L24/28, as all chars, probably exception Solo Mage, Solo Psionic + party combinations of these 2 classes, are immortal, and do sufficient damage for a fluent game.
In another post you asked about Fairie Ninja/Bishop, if I didn't confuse things. At L24, you will never wish to change one of these guys into sth else. Problem is to get them to L10/15.

sometimes yes;
if the main class is finished or at least you have the things you need then you can change class for the end game
a new class that opens new spells like a Bishop to Psionic is common one because they have immune to Mind effects and a lot of late game battles have these ... a Bishop may only have a few Psionic spells so this swap helps

but far above the most common reason is that a cursed weapon like Plague Axe so a Priest needs Valkryie or of course the Cane of Corpus could talk a Faerie into trying Ninja then the Black Gown of Death may be enough to swap a Valkryie to a Lord... assumes she doesn't want to Axe because Lord can't use it ;)

p.s, the Mindblast Rod is a good reason that Bishop or Psionic swap to Monk... not cursed, no fight needed (there is a tick to getting it without upsetting the bugs) and always in the same spot every game
I tend to grab it around level 8 but 10 is a safe level if you are still learning the game so its not really end game area but a solid polearm and we carry it for the rest of the game