Asking questions is good. you want to learn something build it.
Loop on 160 is a full wave length. Big honking antenna.
What is the impedance of that loop antenna on 1.8mhz? What is the impedance on the other bands of that loop?
This is what you have to consider when using a loop antenna.
The internal tuner on that FT890 will not match all bands, no exceptions.
Also you need to feed that loop with ladder to minimize the losses in your transmission line.
The loop on the higher bands will give you some gain, in what direction that gain is in can not be predicted, build it and try it.
Do yourself some studying, get an antenna handbook, get the reflections book so you can understand how transmission lines work.
Used antenna tuners can be had for around $200.00 or less. Or build your own.
If you want a multiband wire antenna with a 50 ohm impedance and use a single coax to feed it then fan dipole is your answer. Will it cover the full band? Yes and no. Depends on what band you want to use, and what portion of the band.
I will assume that CW portion will not be used so for 40 meters and up Yes you can make a fan dipole to cover the full phone portion of each band with exception of maybe 10 meters.
Your internal antenna tuner will touch up the impedance mismatch and allow you to work the full 10 meter band with some loss in the transmission line.
You are asking some good questions, but there is no silver bullet one size fits all antenna.
Experiment, build some of the antennas and see how they work for you. Nice thing about wires is if you do not like the performance you can always make another antenna using the same wire.
The antenna is 95% of your station, you also have to get that thing up in the air to get some performance out of it.
