At the risk of telling you what you already know.

On HF, any wire antenna, even one that is random length and pulled in by ATU, will not benefit much from fat wire. Anything more than about 0.75mm2 will be self supporting unless you want to dangle a big box with a trap in the middle of it mid span. Ends of dipole antennas can come down to be a few feet above ground before switching from wire to insulating cord, and form an  "inverted V" so you only need a single central pole, that can be metal or insulating, but more importantly, quite thin and guyed. Trees work well, so long as the wire elements do not rub the branches, and the fixings at the other and can be arranged to accept the movement in the wind.

In terms of stealth, for the UK, white grey or black PVC insulation is are more or  less as good as each other against the typical (summer) sky.

If you have planning issues, I presume you have neighbours who are not on-side ? If you are on good terms and are able to set peoples minds at rest that they are not all going to be fried or struck by lightning and invite them to see it in action, this can help a lot.

In adversity, then there are 2 approaches, either it it is there for less than 28 days in the calandar year, so it needs no permission (ideal for trying out different designs quickly, a sort of guerrilla approach, bit like lots of field days or JOTAs), or if you can,  putting the antenna up, taking a dated photo with something that ages (small children are good) in the background as proof, and hoping no one complains in the first 4 years. After that has expired, so long as you do not take it down or change it substantially, you are in the clear. Some local authorities are more keen than others on enforcement, and some need reminding of the 4 year rule, but it is there. Note that  ff you are an RSGB member there is quite a bit of planning advice on-line on the members site.

Anything working against an earth electrode needs serious thought about managing the return currents, as the earth will be 'hot' during transmission,  you are right to consider EMC as a possible issue, also bear in mind ground resistance losses can take a significant chunk out  of your useful TX power.

In a high QRM situation, many receivers do better with an attenuator up front - it seems counter intuitive, but generally communications receivers  are optimised for weak signals, and overloading input stages designed for micro-volt levels just leads to odd mixing effects in the stages prior to the channel filtering - and once mixed in, it cannot be unmixed.

At the risk of telling you what you already know.

On HF, any wire antenna, even one that is random length and pulled in by ATU, will not benefit much from fat wire. Anything more than about 0.75mm2 will be self supporting unless you want to dangle a big box with a trap in the middle of it mid span. Ends of dipole antennas can come down to be a few feet above ground before switching from wire to insulating cord, and form an  "inverted V" so you only need a single central pole, that can be metal or insulating, but more importantly, quite thin and guyed. Trees work well, so long as the wire elements do not rub the branches, and the fixings at the other and can be arranged to accept the movement in the wind.

In terms of stealth, for the UK, white grey or black PVC insulation is are more or  less as good as each other against the typical (summer) sky.

If you have planning issues, I presume you have neighbours who are not on-side ? If you are on good terms and are able to set peoples minds at rest that they are not all going to be fried or struck by lightning and invite them to see it in action, this can help a lot.

In adversity, then there are 2 approaches, either it it is there for less than 28 days in the calandar year, so it needs no permission (ideal for trying out different designs quickly, a sort of guerrilla approach, bit like lots of field days or JOTAs), or if you can,  putting the antenna up, taking a dated photo with something that ages (small children are good) in the background as proof, and hoping no one complains in the first 4 years. After that has expired, so long as you do not take it down or change it substantially, you are in the clear. Some local authorities are more keen than others on enforcement, and some need reminding of the 4 year rule, but it is there. Note that  ff you are an RSGB member there is quite a bit of planning advice on-line on the members site.

Anything working against an earth electrode needs serious thought about managing the return currents, as the earth will be 'hot' during transmission,  you are right to consider EMC as a possible issue, also bear in mind ground resistance losses can take a significant chunk out  of your useful TX power.

In a high QRM situation, many receivers do better with an attenuator up front - it seems counter intuitive, but generally communications receivers  are optimised for weak signals, and overloading input stages designed for micro-volt levels just leads to odd mixing effects in the stages prior to the channel filtering - and once mixed in, it cannot be unmixed.