I think this is a very interesting discussion to launch and looking forward to see other members' thoughts.

First of all, there indeed are boards where splitting our range into two sizings doesn't happen. From my viewpoint this often happens in 3bet pots because the SPR i.e. the money left to play is much lower and you can even get all the money in by the river starting with a small size only. Also ranges are usually narrower in these lower SPR pots due to more aggressive preflop action which makes splitting the small range tougher.

But there are also single raised pot situations where betting range with almost only a small sizing is the highest EV line. But in most cases it's just a simplification and at least a small portion of the range wants to use another sizing.

Of course the most obvious reason for betting bigger is that you have some very high equity value hands in your range which want to shovel as much money in the pot as possible. Will the exclusion of these hands make your small betting range weaker and harder to defend? Yes absolutely. But if performed well it will still overall yield a higher EV just because you gain so much more when villain has a bluffcatcher vs your strong hand. But you need to do a good job at
1) Finding the appropriate bluffs that accompany your strong holdings in the big betting range
(e.g. high equity bluffs / bluffs with great blockers)
2) Putting some appropriate still very strong hands into the small sizing (but of course you are right that this alone can't strengthen your small betting range enough)
3) Rely on great bluffcatchers in the small betting range that need to call down

Combining 2+3 should then do a good enough job of preparing you against opponents trying to apply pressure to your small sizings. Regarding 2) when playing around with solvers you will often see that on a K74cch board a set of 4s will use the big size and a set of 7s will favor the big size but still bet small (or also check) some amount of the time. KK often will favor the small size or even check just because it blocks so many continues of our opponent and therefore rather wants to be more cautious and sneakily force our opponent to continue. And the same happens for two pairs, top pairs and so on. So you will always need to ask yourself which hands exactly have better reasons to bet bigger and which can still bet small. By doing this you automatically protect your small bet range.

In the example you outlined point number 3) is not as clear but it becomes much more important when splitting your range into checks and bets. In these cases you will often find that you are forced to call down with a decent top pair even when facing lots of pressure because it might be among your best hands left in your range.

Overall the challenges you mentioned are true and splitting a range into checking and betting or also into small betting and big betting makes the weaker one of the lines more vulnerable. But the advantages you will gain by winning more with your strong hands will overcome the downsides especially if you learn to properly defend and protect the weaker range.

Great answer!
Here the thing, We need to do this very well or we'll lose a lot of chips. The EV we gain by splitting our range will be crushed by the Ev we'll lose by betting the wrong hand with the wrong sizing. And I don't know where do I begin...

If you feel villain is over-bluff raising to try exploit you, you actually have a very good bluff catcher to call down with. Villain has to find value hands to check-raise with too otherwise he is over-bluffing.

Get some hands in and start worrying about this if its actually happening to you in your sessions.