Chris Edwards, I may have heard of the chap. He of the caustic innuendo

I'll ask around here a little more before troubling him. Apparently the issue is that my Amigas 15Khz out video is too low a frequency for my VGA (LED) monitors to handle. I guess I would need one of those (rare) Multisync monitors if I wanted to go that rout, which I do. Another option is to get a Scandoubler in the mix. You know its a real pity those RGB to HDMI cards (which I have in my A2000) do not also output scan-doubled VGA. But then I guess that would be too much for its Pi to process.

I used to run a 10 line BBS system based on Skyline in the Houston area from 1989-2001 called ACME BBS.  Was a good time and a good system to be sure.

What I did was use a RGB to VGA (Buffered I think) and then used a VGA to VGA cable. This then allowed the native 15khz Amiga picture to be sent to the P5 Input connector for it to be mixed and Scandoubled to 31khz for a newer Monitor.
When switching on the Amiga it would output the Kickstart screen.
Yes, this is where I am having major issues. I am not an electronics person by trade or hobby, I just dabble. The diagram on one of those websites shows the input of the 9pin port, and AFAIK you should be able to connect that to the (Analogue?)RGB of the Amiga DB23 port. I have made a cable that does this + sync signals and also ground.
So far nothing, I only get the two separate displays. Its either Piccolo modes via its VGA (very nice) on a VGA monitor OR native Amiga modes via HDMI (I have another card for this) and another monitor, which of course is a bit of a PITA.

I see the piccolo has no Scandoubler? So you definitely need a 15khz capable monitor.
AFAIK I dont think flicker fixing will be a practical issue for me. I'll only be running non-interlaced Amiga modes, for games mainly and then the Piccolo will do Workbench and all of that in modes like 800x600x32bit

Obviously to use the actual RTG side, you need to setup a workbench/AmigaOS and use either Cybergraphx or Picasso96 (P96).
This then allows you to then use the RTG side.
Yes, I have done this and it is working. Took me a little to get used to the Picasso96 mode maker, but eventually got the hang of it. I CAN see modes like 800x600x32bit on a VGA monitor, but its the pass-through cable thats foxing me.
REALLY wished someone out there with a Piccolo and using such a cable could chime in....

So I have just got around to making the required passthough cable for my Piccollo SD64. I installed it yesterday and its custom modes are working, phew.
But I really NEED to get a passthrough cable made as going between the native Amiga modes and Piccolo modes using monitor input swapping is a PITA.

I could do with some further guidance as I have never owned a RTG card before, and the passthrough aspect of it is unfamiliar.
Am I right in thinking that when the passthrough cable is connected and wired correctly, from the Amiga's DB23 to the Piccolo's DB9 that I should see video BOTH when the Amiga is using a native display mode AND also when its in a Piccolo mode and these both come out of the Piccolo VGA port?
In which case I can just use the VGA out on the card and I am good?
There are no jumpers needed for this to work? Its just plugin and off we go?

I am basing my cable from this page https://www.amigawiki.org/doku.php?id=de:expansion:av:piccolo_sd64 and online diagrams of the Amigas video out port.
For my cable, here are my pin assignments on the DB23 and how they connect to the DB9 as shown in the diagram on the URL you linked to above.

Amiga DB23.                                Piccolo DB9
3 - Red Analogue                          -> 1 Red
4 - Green Analogue                       -> 2 Green
5 - Blue Analogue                         -> Blue

11 - Horizontal Sync                      -> 4 Horizontal Sync
12 - Vertical Sync                          -> 5 Vertical Sync

16 - Red Ground                           -> 6 Red Ground
17 - Green/Blue Ground                -> 8 Red Ground (same wire)
18 - Green/Blue Ground                -> 8 Red Ground (same wire)
19 - Sync Ground                         -> 9 Sync Ground

I have found that if I connect the Amigas DB23 pin 11(horizontal Sync), to pin 4 on the DB9(horizontal Sync), the Amiga wont boot. All other pins connected, it WILL boot, but I do not get any passthrough image through the card.
Out of desperation, I have tried connecting the wires to the DB9 both ways, with red on its top left OR top right and all other connections mirrored.
No dice so far and the DB9 is taking beating. I have it currently wired as per the amigawiki guide, with pin11(horizontal Sync) not connected, or as I say, Amiga wont boot.

Is there a bare minimum signal set I can connect to get ANY kind of passthrough working, I don't care if its mono or flickering, just ANY kind of image so I know I have the correct orientation and then I can wire in the rest.

YES! That is it!  I had no memory of a dongle for x-copy.  Thanks to your reply I looked up X-copy and found images of the dongle.  I had searched for JUBA and GEFI, but failed to search for CACHET, which would have answered the question.  All three of those words are written on the board.  After no success with the first two, I assumed they were all board manufacturer marks.  So I guess I need to cut up a db25 connector to make a cable for my external drives.   For some reason I had thrown away the two floppies, case, and cable from my home made dual external drive, but keep the PCB I made for it.  No clue why I did that.  I also tossed my Amax board, mac floppy, and my backup tape drive.  At least I still have all my floppies, and the 2000.  Thanks again.