Interesting. Of curiosity, what would the meta reason be behind making Primal Saiyan less powerful than SSJ 2 or 3? The in-universe justifications you provide make a great deal of sense to me, but I'm curious as to why you want it to be a sidegrade as opposed to a straight upgrade.
Because it's impure. It's not
natural in the same way that the Super Saiyan transformation is (Super Saiyan becomes inevitable for any Saiyan who becomes strong enough, as long as they're given the right 'Switch' to activate it. The next two levels largely work the same way). The two known God variants simply start the progression from scratch, just introducing Godly Ki instead of mortal energies.
Primal Saiyan meanwhile is not. You have to master a berserk state enough to be n full control, you need to then apply the explosive power increase that Super Saiyan provides--all while remaining in control of your instincts.
And then, at the height of your power and strength, you need to
compress it back down to a humanoid form.
Any Ki boosts that you'd get out of this are immediately lost simply
holding it together rather than radiating outward into making you stronger.
The advantage? A Primal Saiyan's vitality and physical strength is
obscene I would say. They may not gain a significantly higher power level, but a tie would pretty much always break in favor of a Primal Saiyan, and it would also give them an edge in stamina--Full Power Super Saiyan may require only a trickle on their energy reserves, but a Primal can go at 100% for hours without getting so much as winded I think, and they'd probably have some damage resistance too. We do know how much of an advantage that
sheer fucking vitality can have even if you're otherwise outmatched--note Freeza basically remained capable of combat until his energy literally tapped out, and the moment that changed, he became ambulatory enough to
continue fighting (Even if it backfired on him). )
Of course, Power Levels remain bullshit, and someone who hits the "Twice your rank" threshold is still likely going to hand you your ass--which makes Super Saiyan 2 and Super Saiyan 3 the optimal progression if you just need more biggatons, because they're vastly easier to learn and master due to their purity. Primal I think is a sidegrade that could only exist in a setting like this, where there are constraints that prevent you from going up to the higher levels. (After all, I think it was that Kid Goku just didn't have the physical stamina to maintain SS3 for more than a minute or two, and SS2 wasn't much better. He used "SS4" because it let him fight with his full and proper skill and strength again, while still providing a solid combat effectiveness boost compared to the basic Super Saiyan state)