A-levels is a pre-U qualification, it's basically the equivalent to STPM for countries like the UK, Singapore etc. basically it's purpose is to get you into an undergraduate programme.

Foundations are basically the same thing but are institution specific, a university will usually have a foundation programme as a pre-u qualification for spm students looking into getting into an undergraduate programme in that university, it's less recognized than general pre-u programmes like A-levels, SAM, STPM, CPU, IBDP etc. since it's institution specific. Being a pre-u graduate basically don't usually put you in any better position to get a job than an SPM graduate, you won't get a difference in pay or anything like that.

Diploma on the other hand, is a qualification that is longer but once you graduate can be used to obtain a job, it's structure is basically about the same as an undergraduate bachelor degree programme but it's a lower academic degree than a Bachelor's Degree. It's usually 2.5-3 years long and once you graduate, if you were to do a bachelor's degree that has similarities to your diploma, you could obtain usually at most a year's worth of exemption.

Doing A-levels definitely will open a lot more doorways to an undergraduate programme all over the world than a Diploma can. Pre-U qualifications were not designed with you going out to work afterwards in mind, they are prepared with the student going towards a Bachelor's Degree.

There's a huge gap between the standards of Foundation and Diploma programmes as it depends on specific institutions, there are some that are tough and can be said maintain certain standards but there are also some which are kinda of a very low standard and you would have to be extremely lazy to fail. Therefore what your aunt said about Diplomas being tougher and of a higher standard is not true. Especially there are some Diploma programmes that were designed to be less academic demanding on the student in our local IPTS since the MQA mandated minimum entry requirement for a Diploma is 3 credits in SPM while Foundation and Pre-u courses are 5 credits so some IPTS just created a Diploma as a longer and easier pathway for a student that did not achieve 5 credits to get into a Bachelor's Degree.

You're doing an LLB, a diploma would have been a waste of your time and money and won't be the most effective pathway for you to enter an LLB anyway. So doing a pre-u like A-levels was the right path there.

Once you obtain you're LLB, you would possess a qualification that is higher than a Diploma and therefore be in a better position than a Diploma graduate. A Bachelor's Degree>Diploma and going through A-levels is the more faster and in your case better pathway to reach that target.

Basically assuming everything else is equal, in terms of getting a job,
Bachelor Degree holder> Diploma holder
A Diploma holder>A-levels holder
Going from Diploma to Bachelor's Degree = Going from Pre-U to Bachelor's Degree

Basically once that qualification that is higher has been obtained, what comes before is almost irrelevant or in some cases like yours, in order to practice law in Malaysia, what qualification you used to get into your LLB will be taken into account if I'm not mistaken.