According to the book, the "pickup tube" is near the bottom, so as to avoid the dirt, water etc. In addition, there is a fuel filter as in the supply line.

I noticed that the fuel tank drain plug is not at the very bottom but slightly higher. Not sure why.

Logically, there is not much difference on the amount of dirt sucked in,
whether the tank level is near empty or not.
1) Heavy particles will stay below the intake/suction point, whatever the level.
2) Light particles that are emulsified and moving around with the fuel will
be the same at the intake/suction point whether the level is low or high.
3) The sloshing of fuel when near empty may not be as strong as at higher level,
so may kick up less dirt, but this is just an assumption, not proven.
4) The fuel return from the carburettor has been filtered, so actually the
system cleans the fuel in the tank by this circulation. So a lower volume in
the tank circulates more times through the filter, so is cleaner. Again just
an assumption, not proven.

Like mentioned by others, we just need to change the filter regularly.

Yes, you are right there.

In my case, I just dripped the oil into the tiny starwheel, as well as the
digits of the tripmeter and odometer.

If the tripmeter was the culprit, I guess this must have freed it up, and
consequently the starwheel and odometer as well, assisted by the
really slippery oil at the same time.

Possibly in my case, the green gear wasn't really slipping in its shaft yet.
Is the green gear driven by the long vertical shaft/gear, in turn driven
by the spinning metal disc (called the speed cup), itself driven by the
speedometer cable ?
The oil is worth giving a try before replacing the whole unit.

Even if you replace the whole speedometer, as in my brother-in-law's case
when he bought a brand new one for his son's Iswara,  the lousy locally-made
thing gave way again after a couple of years. Cost him 140 bucks.

VDO, even the original German-made one, is a lousy brand.

In the 70s, these Toong Fong Mercedes buses had German VDO meters.

As I sat in the front seat, I watched the fuel meter swing wildly to/fro left right.
Like a cheap China toy.

It was horizontally mounted at the bottom of the speedometer, I remember.

I wrote a letter to VDO Germany and their factory in Penang screwing them up
for poor QC then.
Compare that to the trip/odo, fuel/temp, speedometer cable in my old Toyota
Corolla. Still going strong after 30 years when I sold it.

The only thing I had changed in that car was the meter's float in the tank.
Bought a Taiwanese one for 20 bucks.

And that fuel/temp meter was very well stabilized, smooth moving, not like
that toy-like VDO.

Otherwise, just use some simple arithmetic, If you don't use the
car much, say just 10,000 km a year, then replace it only
after seven to eight years.

It won't take more than two hours to dismantle the meter panel,
turn the odometer gear by hand a bit, drip silicone oil in there,
then assemble back.

If you can't find the time, then that can't be helped.

When was the last time, roughly, when the odometer stopped working ?
Just estimate the number of months ago.

Then to be safe, overestimate the distance you drive daily.

Say usually 50 km, or 30 miles,  but put it at 60 to 70 km, or 40 miles.

Multiply by 30 days, then the number of months.
Add that to the mileage already done since you changed the belt
to the last reading of the odometer.

I experience this when, during letting go of the clutch pedal, I hold at the biting position. If I just let go continuously, it does not occur.

Drain it out.

for my case, i always put extra, add more than max line a bit, until now there is no problem. because lazy to drain out and put back again.