There's really no such thing as must-have tool or kit from Festool. Countless woodworkers in the world do great stuff without owning or using any Festool tools, and that supports my first statement.
The only exception is the Kapex...at least according to this fellow who can't sleep without one!
(The guy's look reminds me of the SAW movies.)
Unless a guy is willing to take the time required to make Mortice and Tennon joints, in any of the several ways to make them happen, a Domino is pretty much a must have. Of course as a hobbyist, that time may not matter, but if you are making money with your work, it does.
Sure, you could drill holes with a brace & bit, then drive screws by hand, but who wants to do that?
This whole topic is basically, "it depends"......on a number of factors.
I worked in the custom department of a large cabinet shop for about 10 years before ever having/using any Festool equipment. My first was a DF500 and it made a huge change in the way I work. A few months later, I got an RO90. Again, big difference compared to the other sanders I had.
Next was MFK700, for solid wood edges on shelves or countertops. I bought a CXS to replace a 12v Bosch drill/ driver/ and right angle drill. One tool in place of three.
I got along fine with just those 4 for about 5 years, until a fire killed them all.
During the recovery, I got a TS55 as part of a huge bundle. It was very handy while a bunch of new stationary equipment arrived and was set-up. The routers were just added because of the ability to use them with the track, then impressed me in other ways too.
At this point, I wouldn't give up any of them. Absolutely necessary? No, but I wouldn't want to go back to the hard way. When you earn your living with them, time and convenience matter.
On a side-note. Duo-doweler, not even close to as versatile.
Lamello Zeta is a fabulous tool, but it is more of a compliment to the Domino than a substitute for it. It has the same limitations as a standard biscuit joiner as far as width.