If I was going to keep the theme here, I would probably only speak about Suikoden II but I'll try to give a decent answer for some of the others.

I have not played Tactics Ogre yet, I still need to finish Peace Walker and lately The Last Story has consumed my life more, so I haven't gotten around to it yet. So I can't really say one way or the other about playing it beyond it being the precursor to FFTactics.

FFTA and FFTA2 are a bit complicated... I'm with you that the game is pretty stripped down, a lost of several stats, no movement abilities, a bunch of the reaction and support skills have been removed for the worst, equipment is overly simplified, and the ability learning system makes this game a completionist nightmare...

Between the two, FFTA has the better storyline but the poorer gameplay. FFTA's main plot revolves around the themes of escapism and I felt the use of children just drove this theme home. It's definitely a Matsuno game (despite only being produced by him) and leaves you wanting to debate and talk about its characters and themes. The gameplay... leaves much to be desired, most of the 300 missions are pretty boring and basic "kill everything" fights, and the limited number of maps due to the GBA's limitations and the fact you often need to replay these missions over and over again, can quickly lead to burnout. It has not aged well I'm afraid. If you play this game, I suggest just sticking to the story missions and only doing the side missions for level grinding.

FFTA2 has an awful storyline and mostly bland cookie cutter cast of characters. The main story is awful with a poorly developed villain, plot threads that are only resolved in optional side missions (Adelle) and some poor pacing. The side missions are a lot better, offering more variety of mission objectives and more involved characters and story focus. There are two storylines that are actually well written and would have been infinitely more interesting than the main plot, the rest are largely more comical and charming. The gameplay also improves in some ways and gets worse in other ways. I am not wild about the Judge rules system in the game as I felt it diminished the purpose it had in FFTA (making the game harder) by largely minimizing consequences. The regenerate MP concept largely kills the mage classes usefulness, slows down combat pacing, and makes the Viera the de facto uber mage race. Yet the clan abilities are nice, the new jobs offer more customization variety, and the missions are actually pretty fun.

Suikoden II has a great political plot about warring nations and two friends finding themselves on opposite sides, despite that, its slow in the beginning cause it has to set up the main conflict. The game does have a castle system, and in fact its one of the best in the series, but it does take awhile for you to obtain it since the plot takes awhile to justify it. Customization is actually pretty simple but there are some interesting things you can do. Its streamlined cause you are using 80 characters and games like Suikoden III and V which have more deep and personal customization systems for the characters will show you how much of a headache that is. This system is designed to keep your options open for party configuration (the main element of the series is using different team set-ups as opposed to sticking to one team). Major customization comes from exploiting characters who have multiple rune slots to use, there are actually a decent amount of really good support runes that can build your characters into some pretty mean fighting machines, magic is still the typical spell charges a la FFI but it makes using the mages more strategic since it will be awhile before you can haphazardly use magic without discretion. Other characters obtain unique runes which you can augment with support runes or make them more versatile with magic runes. You can also embed magic runes into weapons and grant special stat boosts or properties(regen effect). The games also introduces the really cool trading economy system where amassing money comes from buying and trading materials. around the City-States. The game also doesn't shoot itself in the foot like Suikoden I and give you an overpowered all enemy kill combo attack... well it does but said character leaves your party eventually, so its only a crutch for the beginning of the game. The duels have been made better and more exciting, though I feel the War battles are not as good as the rest of the series but if you haven't really played the others then this will not be an issue. Honestly the story is the main draw, but I feel Suikoden II has solid gameplay and I like the variety of gameplay elements like duels and War battles, but if you are looking for a deep customization system style game with heavy min/max elements, you will not find it in Suikoden.