Your thinking is ok but your facts are bit off. First, the universal gravity constant G is not changing. The reason for the accelerating expansion is not because of a weakening gravity but because of dark energy, which provides a force against gravity. Before dark energy was confirmed it could have been that expansion was increasing because of a weakening gravity force. It is now known that it is because of dark energy.
The fields in particle physics are infinite entities so there is no need for anything to “stretch”. The Higgs field exists at every point in space.
You cannot assume that if a field is “stretched” (if possible for the particular field) that this would mean a reduction in the effect of the field. In fact for fields with asymptotic freedom the more force used against the field the greater the field strength becomes – to infinity.
This is how the strong force works. While you are not stretching the field, when you attempt to separate the quarks in a nucleon, the farther you pull them apart the stronger the strong force becomes. It is not possible to completely separate them.
The concept of the Higgs field can be bit difficult to discuss as there is a good bit of information one needs to have it make much sense. For example what the Higgs field actually does is to break what is known as Yangs-Mills symmetry. The Yang-Mills field is a basic multi-component field (24 component fields). In breaking this symmetry it causes fermions to gain mass but not gluons as they are not affected by the Higgs field. As you can imagine that leaves out a LOT of detail.
It is also interesting to note that the concept that Higgs had was first seen as only providing a mathematical process that allowed deep analysis of Yang-Mills and other “gauge” symmetries. It was only later that it was determined that the concept could be used to answer one of the most fundamental questions in physics: where does mass come from?
As this is your first post I would point out the “Ask the Experts” is a marketing label not a true evaluation of the type of responses you can expect here.