they had this weird theory that slavery needed to expand to survive and it exhausted the soil or something.

They figured if slavery failed to expand to newly “acquired” territories then slave states would no longer have a legislative majority and slavery would be able to be abolished legislatively without recourse to a civil war. They were wrong about this because the slave aristocracy’s desire to pursue their material interests outweighed their desire to remain in the union. So keep in mind that the “free soil” path to abolishing slavery “without civil war” required genocidal war against natives and continued westward expansion. This is shitty and white supremacist.

I’d hardly call figures like Sumner and Thaddeus Stevens ‘fringe Republicans’, they were major national figures

Thaddeus Stevens and his self-styled “Radical Republican” coalition were absolutely considered politically fringe at the time. The party moderates were free-soilers like Lincoln. Lincoln represented the face of the party for a reason.

https://en.wikipedia.org/wiki/Radical_Republicans

The Radical Republicans (later also known as "Stalwarts") were a faction within the Republican Party, originating from the party's founding in 1854, some 6 years before the Civil War, until the Compromise of 1877, which effectively ended Reconstruction. They called themselves "Radicals" because of their goal of immediate, complete, and permanent eradication of slavery, *without compromise*. They were opposed during the War by the Moderate Republicans (led by President Abraham Lincoln), and by the pro-slavery and anti-Reconstruction Democratic Party. The Radicals were heavily influenced by religious ideals, and many were Christian reformers who saw slavery as evil and the Civil War as God's punishment for slavery

The Radical Republicans opposed Lincoln's terms for reuniting the United States during Reconstruction (1863), which they viewed as too lenient. They proposed an "ironclad oath" that would prevent anyone who supported the Confederacy from voting in Southern elections, but Lincoln blocked it and once Radicals passed the Wade–Davis Bill in 1864, Lincoln vetoed it. The Radicals demanded a more aggressive prosecution of the war, a faster end to slavery and total destruction of the Confederacy. After the war, the Radicals controlled the Joint Committee on Reconstruction.

the Emancipation Proclamation was always a ‘strategic move’.

Yes.

it never had anything to do with freeing slaves,

From ‘Reconstruction: America’s Unfinished Revolution’ by Eric Foner:

Excluded from its purview were the 450,000 slaves in Delaware, Kentucky, Maryland, and Missouri (border slave states that remained within the Union), 275,000 in Union-occupied Tennessee, and tens of thousands more in portions of Louisiana and Virginia under the control of federal armies. But, the Proclamation decreed, the remainder of the nation’s slave population, well over 3 million men, women, and children, “are and henceforth shall be free.”

it notoriously freed basically no slaves.

it legally freed slaves in regions that were in rebellion. the scope of enforcement was weak obviously since the South could only be compelled to follow this by force of arms. But slaves found out about the Proclamation through word of mouth and it inspired many to run away from their plantations and flee to the North or join the union army. This is especially the case since the plantations were poorly guarded with the masters and overseers and other white men mostly drafted into the war.

Also from Foner:

Even in the heart of the Confederacy, far from Union lines, the conflict undermined the South’s “peculiar institution.” Their “grapevine telegraph” kept many slaves remarkably well informed about the war’s progress. In one part of Mississippi, slaves even organized Lincoln’s Legal Loyal League to spread word of the Emancipation Proclamation. Southern armies impressed tens of thousands of slaves into service as laborers, taking them far from their home plantations, offering opportunities for escape, and widening the horizons of those who returned home. The drain of white men into military service left plantations under the control of planters’ wives and elderly and infirm men, whose authority slaves increasingly felt able to challenge. Reports of “demoralized” and “insubordinate” behavior multiplied throughout the South.

it was a political move to make the war about slavery and largely succeeded in that right.

If it was only a political move to make the war about slavery then it would have not provided special exemptions to slave states that were loyal to the union. But as you and I have both established, it was both a political and a strategic move. But it also showed the ideological limits of the “good guys.” In many ways it was “too little too late.” The Union had all the leverage when it won the war. It could have gone out of its way to forbid the type of racist prison slavery we have in the US today, and it could have told the loyal slave states to suck it up and deal with emancipation.

no one had any idea Andrew Johnson would have been so awful. he was most famous before becoming president for saying “treason must be made odious and traitors punished” and railing against the ‘slavocracy’. the Radical Republicans were actually excited at first when Johnson was set to take over for those reasons.

I don’t buy this. These men are politicians and were much more educated and cynical than this. They did not merely take each others’ rhetoric for granted. Andrew Johnson was an on-again off-again slave owner. His record of sometimes owning slaves directly contradicted his populist rhetoric and presented a clear conflict of interest. Lincoln should not have felt the need to pick a Democrat VP. The union had all the leverage after the war was won. There was no reason for these “National Unity” civility gestures. He should have picked a real radical Republican as his VP. That would have made assassinating him a much bigger mistake for the South.