What is the actual, idiomatic way of expressing what I want to say?
- Nous faisons/remplissons nos sacs comme nous le souhaitons.
- On fait nos sacs aussi lourds qu' on le souhaite.
- Nous remplissons nos sacs d'autant d'affaires que nous le souhaitons.
- On met autant d'affaires dans nos sacs qu' on le souhaite.
These are only a few examples, I can add more if needed. By the way, I believe the most accurate way to say litterally what you mean would be :
- Nous faisons nos sacs de telle manière à ce qu'ils soient aussi lourds que nous le voulons.
- Nous remplissons nos sacs d'autant d'affaires que nous souhaitons qu'ils soient lourds.
But it really does not feel right to me : the first one is fine in a written conversation and okay orally speaking (but still not heard much), but the second one sounds like a dissonance to me (i.e. it sounds wrong even though it is grammatically correct).
May "autant" be used for both "as heavy as" as well as "as often as"?
No it cannot. "Autant" can be used for an enumerable quantity, "affaires" in this case :
- J'ai autant d'affaires que toi. ("I have as much things as you.")
- Je prends autant de pâtes que lui. ("I'll have as much pasta as him.")
- Nous aurons autant de temps que vous pour terminer le défi. ("We have as much time to complete the challenge as you.")
But for an adjective, "lourd" here, it is not grammatically correct to use it. We would use "aussi" or "comme" (for the most common cases) :
- Mon sac est aussi lourd que le tien. ("My bag is as heavy as yours.")
- Mon frère est aussi grand que moi. ("My brother is as tall as you.")
- Vous n'êtes pas riches comme eux le sont. ("You are not as rich as they are.")
- Prenez autant de temps que vous voulez. ("Take as much time as you need.")
It is the same difference between "as many as" and "as much as" : one is for enumerable quantities, the other for non-enumerable quantities :
- As many people / As many times / As many potatoes
- As much time / As much soup / As much blood
Point related to the meanings of lourd, and associated expressions :
- When speaking about someone, Il est aussi lourd que toi. ("He is as heavy as you.") can have several meanings : "You both weight the same." (literal) or "He is as annoying as you." (figurative).
- Il supporte un gros/lourd bagage émotionnel. ("He carries a heavy emotional baggage."), Elle s'est constituée un solide bagage de compétences pour son travail. ("She accumulated a good amount of skills for her work."), etc. Here, the [adjective + baggage] group means "a whole lot of", and the positive/negative variation depends on the adjective, lourd ("heavy") being then understood as "overwhelming", "exhausting", or even "devastating".
- Recently, a new use of "lourd" has been observed (amoung the youth, mostly), here are some daily examples : "Ce film est super lourd !" ("This movie is super heavy!"), "Lourd de fou, ton outfit !" ("Your outfit is crazy heavy !"). We could respectively translate those as "This movie is so good!" and "You are so nicely dressed up!". We are looking here at a very positive use of the adjective, to describe something that is very highly appreciated.
- Il subira une opération chirurgicale assez lourde. ("He will undergo a heavy surgery operation.") where lourd.e can be replaced by important, thus meaning "He will undergo major surgery.".