Agreed that retrofitting a power liftgate would be too intensive to be practical. The LXs were fitted with power gates at the start of the 200-series.
In regards to 2016 to 2018 differences, I'm not sure exactly. The later model year LCs in particular, got more and more flowed down from the LX.
I don't have years when these features may have flowed down, but perhaps worthwhile to look over to see what may be important to you:
LX features:
1) Substantially more sound deadening
2) Powered rear hatch for open/close
3) Soft open rear lower tailgate, with power latching
4) Surround camera system
5) Ventilated/Heated front row
6) Mark Levinson Reference (only other model to get the Reference system is the LS) w/surround
7) Power side view mirrors with auto-fold (I believe later LC's go this)
8) AHC - AVS, lift on demand, auto and multi-step damping, anti-dive/roll/squat, leveling, etc., etc.
9) AHC underbody armor
10) Shorter roof rack (LC has a longer roof rack with 3 supports per side)
11) Steering headlights (AFS)
12) Projector headlights ('16+ LCs got this)
13) Headlight washers
14) Storage in rear seat armrest
15) Power slide rear seats
16) Second row storage on front seatbacks is a panel vs net
17) 14-way adjustable driver seat vs 10-way LC (passenger 12-way vs 8-way)
18) Power stowage for 3rd row seat
19) All windows down by holding remote door open button for 3+ seconds (not sure Toyota's have this?)
20) Substantial Lexus key fob (LC uses same one as corolla)
21) Fade-To-Off interior lighting (I believe LC got this in 2014?)
22) Puddle and side step lights
23) Door sil lighting
24) Interior ambient lighting
25) Additional trunk side light and open tailgate lighting
26) +2hp / +2tq (requires 91 octane, LC is 87 octane)
27) More luxurious interior appointments
28) Ventilated outboard rear seats
29) Four-zone climate concierge
30) LED Headlamp with AHS Sequential Turn
31) LED Cornering Lamp
32) 12.3-inch high-res split-screen display (LC 9")
33) Head-up display
34) All-Speed Dynamic Radar Cruise Control (LC DRCC for 25mph and up)
35) Steering wheel paddle shifter
36) Door puddle projector lamps
37) Rear door retractable sun shades
38) 2020 gets special trims: "LX570 Sport" to "LC Heritage"
39) Dual screen rear entertainment
40) Rain sensor wipers