I didn't play Extella Link and I like Samurai Remnant's gameplay much better, but really FSR and Extella are very different games.
While Extella was way more faithful to the standard musou format in that you didn't play as one character the majority of the time and it featured sprawling battle maps that you ran through and completed various objectives on rather than FSR's mini arenas for combat sections, it also had far less depth where no enemy posed a threat to you beyond the final boss of a route. Even enemy servants were simply tankier punching bags than the usual digital mook army, and an optional superboss that you were supposed to defang rather than face head-on like Sunlit Gawain was just way beefier but could be mauled the same as anything else.
Here's how Extella worked: You picked a character from the main menu to then play their route. The main story routes were Nero, Tamamo, Altera, then Nero again, but every character from Karna to Medusa also had mini routes you could play through. The game split the entire cast into 3 factions and 1 rogue: Nero's faction, Tamamo's, Altera's, and the secret character: Artoria, then these factions would determine your forces on a level.
A level would be a fairly expansive map (something like the size of a town on Samurai Remnant) littered with both enemy forces and your own. Your allied Servants would be all across the battlefield attempting to push into territory controlled by the enemy, or defend your parts of the map from invasion. While you usually just wanted to control the entire map, certain levels often gave you other objectives, such as defeating Gawain (the adviser/ battle leader of Nero's forces) who would be able to easily overpower all of your allies on his own.
This kind of faction-based combat was both the highlight of Extella and easily its biggest difference from FSR. When fighting Gawain for example as Tamamo's faction, Karna (the battle leader of Tamamo's forces) would task Lu Bu with delaying the solar-powered gawain for as long as possible while Tamamo would try to cut off all access to sunlight. You could come across them battling when travelling the map and Karna would push you to keep going and not let Lu Bu's sacrifice be in vain. When Lu Bu would inevitably fall, Medusa would travel to his location and take his place to buy you more time, until you were ready to face the depowered Gawain yourself.
This faction system allowed for some great and exciting variety to clearing levels. Fighting Tamamo's faction as Altera for example, would sometimes cause Karna himself to appear. Gilgamesh (the leader of Altera's forces) then yells in Altera's ear to get the fuck out, as even he would struggle against an enemy of that level, and the objective turns to escaping instead.
Additionally, playing as Nero and Tamamo, you could also access a transformation power through their wedding rings with Hakuno, in case you weren't plowing through enough enemies already. Noble Phantasms could also be used by collecting Noble Phantasm chips hidden throughout a level, where collecting 3 of them let you activate an NP.
Unfortunately Extella's puddle-depth combat really dragged down the setup. You can face sunlit Gawain yourself without depowering him, as he's just very tanky but not really dangerous as long as you keep mashing attack buttons. Same thing with Karna. You could go the whole game without ever activating a noble phantasm because these didn't do that much damage compared to simply facerolling the enemy with attacks they didn't defend themselves very well against. While it was really cool to have a map open all the time showing how the battle was progressing throughout the stage, the locations of all your servant allies, and seeing multiple servant battles occur at once, you were always a wrecking ball that had no issues fighting anything so stakes never felt as intense as they could be.
If there's anything Extella's combat actually did well though, it was making the original Saber feel like a badass again. Unlockable only via a secret method and the only character without a faction, Artoria's route was that she was summoned to end the three-way war all on her own, so you would spawn into stages with the entire map against you, but you were an unstoppable force of nature expected to take on multiple servants at once without any backup. There was even a funny level where Saber would take revenge for Fate/Zero, ambushing both Gilgamesh and Iskandar to take out two of Altera's generals at once.
Fate/Samurai Remnant is a much different game with only the same musou foundation built on it. Unlike Extella, you mostly play as a human Iori with multiple fighting styles and an expanded moveset, with servants being the (much more powerful) tag-in allies. Saber is your constant companion that follows you everywhere, and much of the game's combat revolves around Iori as a weak human fighting enemies much stronger than himself, only evening the odds by Saber's aid from combined attacks, commanding Saber to do certain techniques, or swapping to controlling Saber when their gauge is full. The game lets you play as other servants, but these are in short missions or side quests that act as pacebreakers from playing as Iori, and combat is focused on mini-arenas throughout an environment where you only need to take down everything onscreen, rather than accomplishing a set of objectives.
BECAUSE FSR is dedicated to the power differential between Iori and his enemies, FSR ends up with much more complex and interactive combat than Extella. While you shouldn't go expecting DMC or Soulslike combat (Nasu I know I'm not Shibusawa Kou but I think a Sekiro style game with Ryougi Shiki will be a cool idea trust me on this), you are expected to actually read enemy movements and dodge their attacks, and you don't have the power to suppress enemies forever under endless blows because of your limited physical strength. While Iori does grow in strength over the course of the game, a lot of Iori's upgrades involve actually increasing his options, allowing him to learn parries, crowd control, joint attacks with Saber, more efficient dodges, extra effects on his attacks etc that let him more fluidly take on dangerous enemies rather than let him overpower them with brute force.
