I don't bother with defense on volcanic planets. Mining worlds are very replaceable.

defense is generally better. Keep in mind the difference in debuff.
Also, defense makes it a tad bit harder to remove pop, not a huge difference but for the low-energy newbies, it can matter.

mining planets themselves are not worth defending for most ppl over 600, thou some might have well made artifact filled ones(ofc, only artifacts that are exclusive mining, like heat seeker, larva stem etc), they'd be defending the artifacts, not the planet.

Rank 1578 here and I disagree.
The higher rank you get and the more planets you get, things just get more an more Credit intensive. Having a healthy mining production and healthy research production (for research contract) is getting more and more important. Mining gets to be even more important with the changes in the EM markets and purger mission.

While those mining planets might be easily replaceable using toxic purifiers, a cost to colonize that has several months return on investment doesnt really make much sense.
I lower prod a slight amount and giving a mining planet decent defense and ensuring it is not feasible for most people who would want to take your planet to have any shot at success.

How frequently would your mining planets be stolen otherwise?
How much production are you losing out overall for the defense?
How much is your colonization cost?

Probably over 50b at his rank.
Of that's the case, assuming 600 points per hourdefended and say 50 more not it would take 3 years to pay the colonization cost above that he would have made anyways.

Its easy to get 40k attack on a massive 18x irradiated and have it make 750+
I find that to be worth it.

It would not be surprising to lose an undefended toxic at least once a month.

In the build I mentioned earlier, switching out 9 spaces used for def that give no prod to make it a pure prod would increase prod by 10%-15%

My colonize is currently 25B and invade 50B.
So basically if you consider how long it would take to offset colonize costs with the mining output of a planet (using 40k Cr / Mineral point as an estimate) a pure prod VL toxic producing 700 MP/hr would take roughly 37 days to pay back 25B.
So, if a toxic gets taken and I replace it, I will break even with no gain if it happens about once a month.

But that doesnt consider that the example of the defended VL toxic puts out about 600 MP/hr
Considering a difference in production of 100 MP/hr, it would take 260 days to recover from each toxic taken and replaced rather than defending and not getting it taken.

The replacement cost vs the production of a single planet is irrelevant. What matters is whether you are losing more by having to replace lost planet OR by losing out the production on ALL toxics you have for defending them.
For example, let's say you have 40 VL toxics. That means at 10% loss each if you defend them as compared to not defend them. That means 4 planet worth, or ~2800 MP/hr. Which by your calculation takes ~9.25 days to make 25B. That means if you go all production and no defense, as long as you lose less than one planet per 10 days, you'd make more overall, counting your losses. That is to say 25B/37*40*10-25B=245B>25B/37*40*0.9*10=243B

Ofc, whether or not you have 40 toxic is another matter, and I won't be counting irradiated because I'd put all my limited mining specific(such as extracted larva, stripcore) artifacts on 18x M irradiated, which makes them worth protecting as it'd be hard to replace(as it'd give more production than standard toxic even with defense). But if you have fewer toxics, you probably would have less stolen over period of time. Overall, it comes down to losing 10% every 40 days or so. I don't defend mine and I certainly lose my toxic at a far lower rate than 10% per 40 days.

Problem with the pure production path is that it's more risky(thou I think at least at your colonization cost it'd be better than defending) and that due to the fact that you maintain a higher production, you'd need more storage.