For the "challenging" stages in Langrisser, you will get 1 shot max bond or not if you didn't play it right. The catch there is to have the correct soldiers combination with heroes (you shouldn't stick to just 1 type of soldiers in Langrisser M, you need to swap them for situations). And the advantage triangles becomes way more and more important the higher level the stages become, so getting used to those rules are important as well. In fact, if you play it right, the hardest stages can still be cleared with SR characters (I even use R in some stages, eg. Leticia).
That's why the "strategy fun" in Langrisser M is way way higher. Weaker tactician can clear those stages with brute force of sheer power alone, while better tactician clears those with way way lower power characters, the catch is all the same in TRPG genre, it all boils down to how much you are willing to read. It takes a lot of reading "per stage".

In short, Bond isn't a problem at all in Langrisser M. For Langrisser M, the most challenging part is the soldier upgrade, equivalent to SoC's Talent upgrade. Each player will need to commit near years of grind to max those. Even though Langrisser M now sell the low level rank ingredients in guild store to help newbie catch up fast, we still need to grind the higher rank ones. For SoC, the original talent tree is less grindy than Langrisser M but you'll still need about a full year of grind to max them (if you spend most of your stamina daily on it), then there's that add-on that you can see in the recent update that require a few years of grind. Well, the difference is, SoC sell more ingredients to their paid players, so the expedition is a little easier there if you actually buy your progress with real cash. While in Langrisser M, certain grind have no expedition options, so paid or free matters not, everyone needs to grind.

The bond upgrade in Langrisser M can be easily done due to we can buy the ingredients from the guild shop. And nope, you don't really get 1 shot without max bond, there's 5 nodes to upgrade normally and at most, only 2 require another character. So you should be able to get up to 8k power without those 2 nodes, and for PvE contents, you should be able to clear them all with 8k power. And most players do PvP once they reached 6k, so it's not really a big deal. You can get to 10k power with everything upgraded though, which is OP due to most character can one shot PvE enemies at 8k power already. As for the "another character" required to unlock bond, actually SoC have something similar, you need another character "alt" to unlock special skill on certain characters, and more and more of these will be coming soon to SoC.

I played Langrisser M for quite a long time but stopped some time ago, so I might be mistaken. However, the bond system is something I never liked. The difference between a hero with fully maxed bonds and one without them is enormous. You yourself mentioned that the challenges are quite difficult, so heroes without maxed bonds will just get one-shot if you try hardest event challenges. That means in most cases, to use a hero in endgame content, you need to summon at least one (or preferably two) of their bond partners to unlock the 4th and 5th bonds.
In SoC bonds just give 5% stats, its nothing compare to Langrisser M

There's bond system in SoC too, and it takes quite some time to max without actually paying for the cash bond item. While in Langrisser you can pile up tons of those ingredients at end game and can straight away max a character as soon as you got their bonding partners. Plus, not all characters need bonding partners too. And they also sell those ingredients in Guild Store where as long as you are active in guild events you can easily buy your way to max a character bond (for free), so it's quite new player friendly as well... But the bond isn't meant to be unlocked at day 1 game play, it's capped by your commander's level (to prevent your characters from being too OP at newbie stages), so to unlock most stuffs, you need to get to a higher level. Both SoC and Langrisser M doesn't "start" until you maxed your commander's level anyway, that's where the real stuffs begins.

You didn’t mention bond system in Langrisser M, but it makes a huge difference. You can pull hero but cant fully use them without unlocking their 4th or 5th bond. This makes game very unfriendly for new players. Still, both games are great.

Judging from the way the game is made and how Masaya treated it as their current main franchise allowing L1~5 + Rebirth + Millennium added to Langrisser M, plus M own's 4 stories, it's now basically the whole history of Langrisser itself in one game. There is a high chance that it will be made into offline version when the contract with Lzong is near an end or after the game server shutting down, they've put too much love in the making to have let it end just like that. Probably ended up as a remake + DLC for the skins here on Steam...

I'd like Langrisser M way more if it was actually like the console games and not a mobile gacha game tbh

Multiplayer Interaction in Langrisser M is more fun. While in SOC is pretty bad and almost none. In Langrisser M have seasonal PVP like Apex Arena. While in SOC, same gameplay pvp mode but not a seasonal PVP and need a 3rd party program such as discord to have a PVP match up with other player, which is pretty annoying

Comparison Chart is still a guide nonetheless for those who wished to know whether SoC is treating its players correctly or whether or not they should start playing the game itself. These information is there to guide the players when making choices. You can link this guide on reddit if you'd like a discussion with others.

Basically SoC copy-cat the core design of Langrisser M and slapped a 2D-HD Octopath theme over it. You will find a lot of commons between the 2 games. But there are still differences, and this guide is here mainly to show what are the difference between the two.