The Walker tariff lowered tariff rates to about 25%. Note that the Confederate Constitution only allowed a tariff for revenue - not a protective tariff. A tariff for revenue allows a maximum of just 10%. So even the "compromise" tariff rate was still orders of magnitude higher than would have been in the best interest of the Southern states.
But of course the Morrill tariff promised to greatly increase tariff rates. It passed the House in 1860 and was just one or two senators short of being able to pass the Senate in 1861. With the usual tactic of allowing a special carve out or some other lucrative benefit to appease the commercial interests in the district of one or two more Senators, it would pass. Of course, there would be pressure on each senator to agree lest somebody else take the deal on offer to him and his state be shut out completely. Lincoln himself was staunchly in favor of a high protectionist tariff. As it turns out, the Morrill Tariff passed and was signed into law before Lincoln took office.
Why would preservation of slavery have been a real concern when there was no real popular support to abolish it and considerable opposition to doing so everywhere as well as no power to do so?
Here’s your maximum of 10% in the Confederate tariff rates.
ACT TO PROVIDE REVENUE FROM COMMODITIES IMPORTED FROM FOREIGN COUNTRIES.
SECTION 1.
The Congress of the Confederate States of America do enact, That from and after the 31st day of August next, a duty shall be imposed on all goods, products, wares and merchandize imported from abroad into the Confederate States of America, as follows:On all articles enumerated in Schedule A, an ad valorem duty of twenty-five per centum. On all articles enumerated in Schedule B, an ad valorem duty of twenty per centum. On all articles enumerated in Schedule C, an ad valorem duty of fifteen per centum. On all articles enumerated in Schedule D, an ad valorem duty of ten per centum. On all articles enumerated in Schedule E, an ad valorem duty of five per centum. And that all articles enumerated in Schedule F, a Specific Duty as therein named. And that all articles enumerated in Schedule G, shall be exempt from duty: to wit:
Your can read the whole thing here.

According to you, a 25% rate is far above what is necessary for revenue. So we’re the confederats protecting domestic industries. And was the war really about Tariff rates when the vast majority of the South’s population never paid a penny in tariffs.
The Morrill Tariff had been bottled up in the Senate by Southern states and would not have passed except for secession. Blaming secession on the Morrill Tariff confuses cause and effect.
Go read the articles of secession by the Southern states. They do not refer to tariffs but repeatedly refer to the protection of slavery as the reason for secession.
With some justification, slaveholders feared that abolitionist agitations would inspire slaves to flee North or to revolt. Slavery required Southern whites to support a strong apparatus of enforcement, control, and repression lest the large slave population turn on them.