I get that you want the stack sizes, and I ain't against that; for you (easy mod in xmls) or anyone else. But it seems it has nothing to do with variation in character builds...? You wouldn't skip it yourself, if it was an option, no?
This is another point of discussion we don't seem to be on the same page on.
Hypothetical "Construction career paths"
1) Increasing stack sizes, slowly and incrementally, over time (all of which yet to be determined)....
2) Building up skill in building 'better blocks'. (think in terms of more expensive recipe, but higher hit point blocks)...
3) Other things, as hitherto undrempth of.

Sorry, I couldn't resist.
You wouldn't be able to choose progress form more than one path, so choosing the one you want to explore is painful, but neither are game changing/breaking.
And of course, I am looking at characters being able to 'switch careers' over time. I had been thinking 1st career at level 100, second at level 200, and third and final at 300, however, folks responding have pointed out that waiting to level 100, might be to long, while others have mentioned getting one right off.
Work in progress, what if"
characters unlock career paths starting at level 50 (up for discussion of course).
1st Career at 50th level.
2nd Career at 100th level.
3rd Career at 150th level.
Possibly more Career paths later on? Each of which has at least two different trees? Out of how many Career paths?
As an aside, you mentioning of "higher level/Hybrid" Careers is good, but lets just concentrate for the moment on the simple career paths.
If there were ten or more career paths within the game, and I could only ever get to walk down 3 of these, and only the one side of those at that, well yes, choices would abound.
When before you said the word "sacrifice", I attempted to convey my dislike of the (perceived) mindset, that one would have to give up something
they otherwise deserved. Everyone getting everything is the definition of cookie-cutter design. No one that is playing with optional setting that eliminates that, should be stuck in the mindset that they are "giving something up", but rather that, they are being offered something new and additional, but in this part of the game, they have to make choices
I get that you want the stack sizes, and I ain't against that; for you (easy mod in xmls) or anyone else. But it seems it has nothing to do with variation in character builds...? You wouldn't skip it yourself, if it was an option, no?
I might. Depends entirely on what I wanted to do, in any particular game. I would always want the stack sizes, but perhaps the other option appeals more?
Say there were 10 total Career Choices under this notional idea.
Say I could only ever get three of these.
Say I can only get, at most, half of any given Career.
Plenty of choices.
Identify the two mindsets that cannot function in such optional setting, and warn them that these setting might not be for them.