With your streaming/downloading i'd recommend more RAM. You probably won't be fully happy performance wise with that board/proc combo but it's a great and inexpensive start and it'll definitely be more sufficient for far less money than most full retail Syno units.
I have 16gb currently and it gets tapped while using SABNZBD to download and RADARR/SONARR to process. I use them in docker containers and can easily see which one is using the resources that way. I'm no expert on this by any means. I've recently built my first Xpenology box. But I may be able to answer some of your questions/concerns.
1) This isn't an issue with how Synologys SHR operates
2)I do this now. I have a reverse proxy setup so I have subdomains pointing to different thigns (tv.domain.com for sonarr, movies.domain.com for radarr, download.domain.com for sab www.domain.com for Web/Wordpress). This has been the hardest thing to do because of how strict i keep my pfsense firewall policies)
3)Easy enough, you may run into some issues initially the more you'll be able to figure out how DSM does things. You can't think of it like a windows share
4)Are you going to use Plex? I'd recommend plex.
5)I used to do this, I switched over to plex. I have the appletv4k plus all ios devices. I don't miss out on anything by not running itunes server realistically because I have the apple music subscription too
6)PUSH notifications are currently not registering for me, and I really haven't put effort into figuring out why. It just won't register
7)As long as your ports are configured currectly this is no issue
8)I can't comment on this one.
9)I used this a lot with legit syno devices at work and with my xpen at home. Works great
I have a legit synology and my xpenology on my home internal network. I just access them with \\littlenas \\bignas in explorer
To map locally iscsi would be best, IMO. YMMV though. iscsi will allow the mapped area to appear as X:\ without having the little green network pipe (where it acts as a mapped drive). I can say this though. When initially setting things up I replaced my C:\Users\username\Downloads location with \\bignas\downloads and windows automagically enabled sync center and used my primary volume of my PC to cache and it ate all the space of my C:\ very quickly. I had to disable that and have since switched to iscsi which loads great each time the PC rebooted, as long as the NAS is running.
I hope the above helps answer some of your questions. Google has been the biggest help. I've only ran into one issue that wasn't solvable by googling, so I wiped the box and started over and that fixed everything.