Just so you know, the Coastal Camino doesn't start until Porto. Until then you will be walking inland.
True, but there are coastal alternatives from Lisbon that seem to link up to Fatima and then Tomar. IIRC,
@AJGuillaume and his wife walked out of Lisbon in such a route recently, which he describes
here. He and his beloved are very experienced pilgrims, and will have a view on whether that would be suitable for a first time pilgrim.
On the question of the CP route, I walked the Caminho do Tejo or Tagus Route from Lisbon to Fatima before crossing (by bus) to Tomar and walking the CP to Santiago. I'm not sure what is meant by 'sketchy' here, but that's not an adjective I would have used.
I broke up a couple of long stages early on by going back and forth on public transport. There were still some places that hadn't reopened after COVID, and perhaps never will. But there were alternatives. Some might have been a bit more expensive than I would have liked, but I never went without a bed. Much of what is written about things like high daytime temperatures, traffic on busy roads and the like is true, but I never found it the difficulty some people make it seem to be.
For the OP, if you have done other long routes, and you are mentally and emotionally prepared to walk alone occasionally if there is no-one else to walk with on some day, I think this route is well worth the effort. If you do start it, and find that you aren't coping well with it, towns along the route are generally well connected by train and bus, and getting up to Porto and walking from there is always going to be an option.