NEC 110.2 says that equipment must be "approved". This means by the local Authority Having Jurisdiction (AHJ, the town electrical inspector).
It is the policy of essentially every electrical inspector to rubber-stamp any fixture which is listed by Underwriter's Laboratories (UL) or other Nationally Recognized Testing Lab (NRTL). Note that CE* is not a testing lab. Neither is FCC, CCC, CCCanything, or RoHS.
No credible NRTL mark and file number, you have to sell it to the AHJ.
18 AWG is fine inside a fixture. I don't suggest trying to put 19 wires under a wire nut, or otherwise abuse/overload connectors.
If you feel comfortable putting six #18s on a wire nut, then have 4 groups of 4-5-5-4 each with a #18 pigtail, then and gather those four into another wire nut with a #14 or #12 pigtail. That pigtails to the ceiling.
Otherwise group 3 wires to a #18 pigtail (six of these), 3 of these pigtails to a #14 or #12 pigtail (two of these), then join those #12s with the ceiling wires.
Yes, all blacks go together and all whites go together, placing them in parallel.
You would not wire the LEDs in series unless you were using bare emitters and a constant-current driver circuit. (Constant-current very much favors series connection). In that case you would never socket them; the LEDs would be hardwired.
* CE stands for "Chinese Excrement". Not really. It's a voluntary self-certification for businesses physically in the EU (where they're sueable if they cheat). Anyone else can slap it on anything with no consequences.