Anyone can have the master tier subscription and enable sharing in the campaign. This will share books between all users, so if you own one book and a player owns another you'll both see both books. So yes you only need one master tier sub but it doesn't necessarily have to be the one who owns the books.

The hero tier allows them to make unlimited characters/encounter, and add public homebrew (published by the community) to their collections. Along with the sub perks and new tools although I don't think there's any current tool locked behind a sub.

All private homebrew any of you or your players create will be automatically shared with or without the master tier.

The big caution is to remember revised core books will be coming out next year. If you think you may switch to the updated rules, it may be better to hold off on the digital purchases. However, if you are definitely going to stick with the current version, or even if you are not sure, then it makes a lot of sense to buy now, as in all likelihood the existing books won’t be available for digital sale when the new book are released. 

You could probably get away with the free Basic Rules & SRD and the CoS adventure module and just pick up whatever subclasses, races, feats, spells, etc. that you want à la carte. That would likely save you money long term.

Online services are at the mercy of the companies providing them.  If they change their policies or otherwise stop providing a service, however much you've invested in it suddenly dries up, and you lose it.  Even if WotC doesn't pull another stunt, if your internet goes out for any number of reasons, you still lose that access, through no fault of your own, and you'll be making ongoing payments to maintain that status quo.

That's why I much prefer pen-and-paper.  Once you've bought it, it's yours, and even if civilization fell, if you had a couple people with time to play, you could still run games.

Also, physical copies can be purchased second-hand, potentially saving you a lot of money, and 3rd party content can be easily added to games without having to struggle with programs to figure out how to incorporate mechanics or spells that aren't WotC creations into your game.

There are advantages and disadvantages. If you are playing remotely you can not share physical books with the other players, If you want to play a drakewarden ranger but have no interest in the rest of FToD you can buy the subclass on its own digitally but not physically. Being able to just click on a spell / feature on my character sheet in D&D beyond to check the details ("can I cast bless on the rogue I can not see?") is so much easier and quicker than rumaging through the book (possibly books if you don't note on the character sheet that Sivery Bards is in Strixhaven).

Yes buying digitally does require a certain level of trust on the seller.  If civilization fell you might be able to play D&D with physical books at least until looters steal them all.

What is best varies from person to person.

Quote from Jegpeg >>

There are advantages and disadvantages. If you are playing remotely you can not share physical books with the other players, If you want to play a drakewarden ranger but have no interest in the rest of FToD you can buy the subclass on its own digitally but not physically. Being able to just click on a spell / feature on my character sheet in D&D beyond to check the details ("can I cast bless on the rogue I can not see?") is so much easier and quicker than rumaging through the book (possibly books if you don't note on the character sheet that Sivery Bards is in Strixhaven).

Yes buying digitally does require a certain level of trust on the seller.  If civilization fell you might be able to play D&D with physical books at least until looters steal them all.

What is best varies from person to person.

 That level of trust in the seller is something I don't have.  This is the company that tried to retract a non-revocable OGL to force people to sign a new license that gave everything the fans created to WotC, so they could sell it, while also giving them the authority to prevent you from making any money off of it.  And they never acknowledged that what they tried to do was wrong on a fundamental level, they made excuses and tried to push it under the rug, hoping the backlash would die down so they could eventually try it again.  That's not a situation that I want to willingly invest in.  It's like buying a car that keeps catching itself on fire.

I know for some people the risks are outweighed by the features of online gaming, but I expect that WotC will burn the players again (just as they have multiple times before), it's just a matter of time, and those online gamers might re-assess their position when that happens.

imo, buy what you need when you need it - not before. You don't need the monster manual if you're running a specific adventure...you might want the monsters IN the monster manual though (its cheaper that way) just because its tempting. As a DM, you also don't need the PHB (or the DMG really)...what you need is already provided for free.

If you want all 4 of those books 'just because'...great....but you don't need them.