Sure Shiptrax looks good.
1. Yes you've got a point, asked several times for it for Shipplotter (further named SP)
2. Another good point, 1 and 2 is much the same point.
3. in SP you can also make alert zones, as many as you like, with many filter options
4. Doesn't have to in SP, you can zoom in and out on maps
5. good point of Shiptrax, but in SP lookup of max distance
6. standard available in SP, you can make colors on thiptype/navigation aids etc., but
also colorcoding on underway, anchored etc.etc.etc.
Did you ever have a good look into SP??
Furthermore:
does Shiptrax also support Inland AIS messages, in early information of Shiptrax is hasn't,
SP has, supports all messages.
So I'm not contra Shiptrax or just pro SP, both have nice features...