I'm at rank 179, making 1992 mineral units per hour and I'm well on my way into researching Light Thetacron Cannons (completing within roughly a week or less) and I am currently equipped with 6 heavy anti proton cannons as my standard weapons.
I was planning on gradually replacing these with Light Thetas (Effectively skipping Null Rays entirely and being ahead of a lot of people my rank) while working on the Mylarai mission chain.

But I was just looking for input on this plan of mine, would it actually be more advisable to use Heavy Null Rays instead or is the idea fine as it is?

Avoid going with light weapons, just wait until you have heavies. You will have a hard time supporting the upkeep with that kind of mineral production, stay with Antiprotons, they aren't that bad. If you must, go with Heavy Nulls.

Nulls definitely.
Should go heavy to heavy.
And although you may be able to support heavy thetas soon I would not recommend switching. Its a small attack boost for a large upkeep increase.
You would be much better off using those credits for planet upgrades.
you can go a long way with nulls. I myself still haven't switched as I'm not 100% on it being worth it.