Replace tubes?

Lightest tubes are stock ktm, they are thin crap.get some ultra heavy duty tubes and forget about punctures.i patch thick tubes and re use unless they have massive holes.

Yes replace the tubes.

I always get the thickest tubes i can find. My fat ass will pinch a lightweight tube in about 5 mins.

2.5mm ront tubes are no issue. Rear, just run a mousse. If not, 3mm and 1.00bar will do it. But traction will suffer compared to a decent mousse (Dunlop or Nitro for example)

Thick (Heavy Duty) tubes are great off-road, but not necessary in MX if you check your tire pressure often. I stay right around 12.5 PSI and almost never have a pinch flat or other tire issue even on the rocky hard-packed tracks.

If your tubes are chafed, you need to replace them. (you'll see a lot of wear on the tube like it's rubbed on the inside of the tire carcass.

A good way to prevent tube chafing is to dump a spoon-full of foot powder into the tire and roll it around to coat the inside before you install it. Rub some foot powder on the tube as well.

Make sure it's talc based foot powder, not starch based. It will say in the ingredients on the back of the bottle.

I’m with you. Fat bike tubes are great as they have lots of excess tube to avoid pinching/tearing. With loads of talcum powder.
I dinged my Sm pro to the point I relaced a new rim to it and it never punctured. Certain is the oversized diameter tube and talcum.

get 1mm tubes. They are the lightest but most likely to get a flat.

Tubes wear out over time, definately replace them occasionally.

As mentioned, look for the chafing damage, little sheds of rubber.

1. Starting with the initial question, no I don't replace tubes every time I replace a tire. Inspect the tube for chaffing (as stated above) and ensure it holds air while it is out of the tire (an air compressor and soapy water help with this). After verifying the tube is good and while you've got it covered in soapy water, clean the thing off and reuse it and spend 12 dollars on more gas to ride.
2. Heavy-duty tubes and Ultra-heavy-duty tubes. These things are typically both a pain to install and add unnecessary weight in the most in-efficient place that weight could be placed. On a standard full size motocross bike, every pound added in rotational weight (ie. wheel, tire, etc.) is roughly 3 pounds of static weight. This both robs power and adds "unsprung weight". Lets call it dead weight. Dead weight = less power transmitted to dirt. Dead weight also = less suspension control. If you're an offroader and need the heavy-duty tubes, absolutely, they have their place in the world. if you're a motocrosser that's looking for the edge over ET3 next weekend, standard tubes will do just fine.

Oem Honda or oem ktm, don't run heavy tubes, it's pointless, if your gona pinch flat, your probably riding a 4 stroke, or not running enough tire pressure. Never once have i ever punched a tube riding, changing it out yes, I think almost erbody has at least one time.

Heavy duty tubes rob power and add rotating mas to your tires making it harder to change directions, and overal make the handling of your bike like poop. If you have trouble with pinching during tire changes, fill your tube, and release air as needed, this will help prevent pinching them with irons, but won't eliminate it, it takes caution and not being a total tool to change tires without issues, make sure your irons contact metal not rubber when your prying a rubber over the lip of the rim.