I in person tried all 3 paths on my customer cars, so can share you a bit of feedback (say dated based on 2020 Apr/May experience):
Retrofitting original AA/ACP: correct hub (really not dual needed I think - just right one - see there is plenty of 2018/19 Giulia&Stelvios with factory AA/ACP and single USB hub) and going through rest of activaion (proxi/Witech) is one option, but it requires 2nd generation headunit, so one which firmware version starts at least from MV051.xxx.xxx (in some documents FCA mentiones MV003.xxx.xxx but such I never seen on the road cars), and then also firmware upgrade to at least MV056.xxx.xxx (but better way newer as early AA/ACP firmwares caused a lot of troubles with some phones - e.g. I had MV059 from factory and plenty of issues with establishing connections with any of 3 phones I had on any cable, where on newer cars with MV072 it was working 100% fine). Aside of need of proxi change (in some cars manufactured in 2018 but without AA/ACP there may be also SGW to bypass first), friendly and willing to help dealer with Witech2 is required to agree to finish activation. Don't go other order in activation as radio will be locked for AA/ACP off forever. Only cable AA/ACP here, and no dual scren support for 8.8 displays (as far, there is petition to FCA maybe something will change one day?).
Eng custom solution. Car I installed it to - intermediate loom to radio went very loose/easily, and we had trouble with disapperaing right channel of AUX (AUX is used to transfer all sound/voice from this add on device so it was real issue) - after bit of playing with connector we got that sorted - looks for good. Firmware seems bit more mature (vs KAP). Problem is call quality - issues with echo very often (various phones used Android and Apple), and voice level on the other side of call was bit too low. We tried also different microphone (Pioneer CRM1083) and tried different installation point for mic (original above rear view mirror, we tried also beside instrument cluster which was worse - quieter but with the same echo). My customer is not fully happy about this one thing, rest is very good.
On Android Auto this device gives dual screen support if one has 8.8 display.
Carplay on WiFi (and USB), AA only cable (Althought I heard recently that there may be update to do AA on WiFi - not sure about it).
KAP Trade solution. late Dec firmwares were incomplete in functions (in Oct ENG had already kind of complete set of features), late Jan seemed to address most of major lacks vs ENG Custom, there were some other issues, and new firmwares released which guy who choose KAP (and I installed it in his GIulia) got from KAP (good support, very quick - here is big plus). Microphone was not the best (Again separate microphone) - too quiet - we used Pioneer CRM1083 instead and all improved and my customer happy. With intermediate radio loom there were issues to plug to radio - contacts must been very tight or not self postinoning right, so it was bit of fight of tilting/pressing/attempts to get it in postion, and even though it finally went it, it was hard to lock the plug on radio, but no contact issues.
On Android Auto, the device works in single screen mode and stretches horizontally for 8.8" displays - looks bit odd (round icons are bit of flat elypses, text is wide, no benefit of wide screen really).
Carplay on WiFi (and USB), AA on USB only.
Eng Custom and KAP advantage is they will be likely upgraded in future, where I expect FCA is likely won't update anything to extend functionality, especially as they deal now with face lifted models likely as only priority..
Choice is not that obvious IMHO, luckily I do not need to make such - my Veloce is with factory AA/ACP, and I hope one day there will be device which I plug to USB and gain AA over WiFi (such solution for ACP exist, so maybe I change to iPhone and buy such?). I raraly need to use AA currently (holiday trip navigation), so I'm also OK to connect via USB in such cases (and keep phone charged), in future maybe guy from xda-developers.com (Emil), will come with 100% working solution for WiFi AA (I used his solution/beta apks for some time with 2nd phone working as AA bridge, but it was not fully stable/reliable, and newer versions which he did completely stopped, and then after phone upgraded to newer android even old betas were newarly impossible to start without crash, so I gave up, but I know he works on off the shelf bridge device to add WiFi AA to cars withtout such (but with USB AA)).