Recipe for a weak concrete mix suitable for patio substrate and similar work
I think you’re confusing “weak concrete” with paving substrate. 

If you use a “weak” concrete mix, it will harden. Then, with any settlement it will crack and pieces will shift. 

However, if you use a “crushed” rock gravel fill, it will bind together yet remain flexible so it doesn’t crack and be displaced over time from loads, etc. 

Both can be removed in the future when your plans change. 

I’d recommend a 3/4” minus crushed rock. This gravel is comprised of various size rock that has been crushed from what can fall through a 3/4” sieve down to sand. These particles will bind together like a “solid rock”, yet will flex. 

When you install the “mix” make sure you compact it in about 6” thick lifts or less.