As a former Auckland resident (and regular across football grounds there) and now resident in Portugal, I feel fairly well-placed to respond to the questions of quality in Portuguese football.
Yup, it's fair to say the "big three" hoover up any major talent that is harvested in Portugal. Money is a huge factor. In short, Portugal is not a wealthy nation and football is included in that. The big three don't pay big wages compared to most leagues in Europe, and the second tier even less so. That said, the quality is good in the top-flight. Boavista are a different kettle of fish because they were relegated to the third tier a few years back (illegally it turned out) and they have only been back in the top-flight for close to three seasons.
Due to various restructures of the pyramid in Portugal, the second tier is due to change again next season. Five get relegated this season to shrink the divisions down. Only two will automatically be promoted from the third tier (a possible third side if they win play-offs against the 17th & 18th in the second tier). Gil Vicente are currently in the process of taking the FPF to court over illegal relegation and look likely to win. If they do, they'll have to be promoted back to the top-flight and then that will require more restructuring.
With regards to Tyler Boyd at Guimarães B, it may be playing for a reserves side in the pure sense, but Guimarães are one of the bigger clubs in Portugal (maybe 4th or 5th) and regularly get five-figure crowds. By being in the club's B team, Boyd has a great opportunity to progress to Pedro Martins' first team. They're flying high in 5th in the top-flight at the moment. He has previously been involved with the first team's squad but is seen as very much a work in progress over here. Portugal may not be a glamour league (especially the second tier) but it is a fantastic place for players to learn their trade and kick on. Scouts come to Portugal from almost all of the major leagues in other countries. Boyd is well positioned.