I have an ICOM IC-7300 which I am currently using for listening to the amateur bands but also general HF listening
Trying to sum it up, skip the idea of the external ATU and use the 7300 built-in one
Set up the antenna as shown in this image (no artwork
!)
keep the ropes between the antenna and the two supports long enough to allow for a spacing of 50cm to 1m (or more), the antenna wire doesn't need to be horizontal, it may be sloping; use a pulley at the far end support and connect that end of the support rope to a weight (it may even be a brick) so that, if the antenna is loaded by wind or ice, the rope will be free to slide and the antenna wire won't break, that's a very old trick, but it still works; optionally you may add a second pulley to the other side of the antenna, that would allow you to lower the whole antenna for manteinance if/when needed, lay the counterpoise down as shown, the counterpoise wire may also be laid down on ground if needed, for the ropes pick some good UV and weather resistant one, the so called "paracord" may fit
At the "near" end of the antenna connect a 9:1 UnUn, following it, connect an 1:1 choke, either directly after the UnUn or just before the coax enters the building, the coax will then go to an ATU or, if your rig has an embedded ATU, directly to the rig, use decent quality coax (I suggest using RG8 at least) and good connectors, stay far away from "chinese" UnUns, Chokes and coax cable, they usually suck !
As for the length of the main antenna wire (the horizontal one in the pic), select it from here
https://udel.edu/~mm/ham/randomWire/picking the longer length which will fit your available space (70ft is the minumum to cover 80-10, 135ft is the minimum for 160-10), try keeping the antenna in a straight line, if possible; if not you may run the wire at some angle (at/above 100°) either/both horizontally and/or vertically.
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whatever you'll decide, please, PLEASE, do NOT use that piece of wire you are calling "longwire" to transmit... and then say bad words at that poor IC7300