Can I make a trip of 260 km across Japan with a pedal assist bike?

Just curious if it sounds crazy to use a PEDAL ASSIST E-Bike to make a trip across Japan...somewhere like 260KMs total

I find that on road e-bikes with around 500 watt-hour battery, and best low rolling resistance tires you can buy (Continental GP 5000 pumped to high pressure), the range varies between 75 and 150 km. 75 km is more realistic when tired and riding in city traffic. 150 km is more realistic when riding in countryside where you have to stop only very rarely, and not being tired.

260 kilometers requires a spare battery (around 3 kilograms) or the ability to stop somewhere to charge the battery (the charger has some weight too even though it's far lighter than 3 kilograms). Even the best e-bikes typically have at least 2.5 hours of charging time from empty to full, and even that might require buying a more powerful charger than the one that came with the bike.

Also what are some good bags (that don't make my back sweat so much)

Avoid backpacks. They make your back sweat a lot. In contrast, a messenger bag causes only very minimal sweat, so little that it doesn't annoy you.

As for bags, I have two types that I use.

First is a messenger bag. I always carry it everywhere, including on the bike and off the bike. Perfect for carrying the mini pump, tools for emergency repairs, spare tube, wallet, mobile phone, keys (you don't want keys in your pocket as the sweat will dissolve metals that may cause allergic reactions), etc. Also the main compartment allows carrying a reasonable amount of stuff when being unprepared and not taking panniers with you. My choice for messenger bags is Marimekko Urbaani, costs around 150 euros/dollars/whatever. It can take any 13 inch laptop and some 14 inch laptops might fit too if they are small enough. Easiest to find in Finland (Marimekko is a Finnish brand), but I'm sure nowadays it can be ordered to other countries online.

I find my messenger bag makes for example saddle bags unneeded. The messenger bag has far more space for stuff.

Second is panniers of good quality. I use Ortlieb Back Roller Classic. They are waterproof if riding in the rain. You want to put the heavy and large stuff there, and the light and small stuff in the messenger bag. You can easily carry a pair of panniers when off the bike using the handles so that the panniers are back to back, using only one of your hands.

pants I should where to make my bum nice and soft

Depends on the saddle. I use non cycling specific sports shorts. I find they work well enough if the saddle has a reasonable amount of padding. Usually when riding for long distances, the area that hurts the most are my feet. So even though they are not cycling specific, I find they don't limit my comfortable cycling distance at all.

However, nobody weighs cycling shorts but everyone weighs their bikes, so it has become trendy to supply bikes with ridiculously lightweight saddles, saddles that have practically no padding so they can be reasonably used only with cycling specific shorts that have a lot of padding. I find the trend ridiculous, as it just moves weight from one place to another place, from a place that is weighed to a place that is not weighed. The total weight probably stays about the same.