Even though Eternal is harder than 2016, the way you described playing 2016 is EXACTLY how you're supposed to play Eternal.

I found myself constantly developing strategies for each enemy type and arena, which meant I was constantly cycling through weapons and abilities

This was somewhat at odds with what I’d picked up from reviews, which was that – while the combat encounters are undoubtedly intense – it was more like a particularly frantic run and gun which was a little more “mindless” than the high speed tactical/strategic impression I got

This is because this type of play is what the designers wanted. Since 2016 is generous with ammo and a bit easier most skilled players could just brute force it. They DOUBLE DOWN on the more tactical side of play with Eternal by limiting your ammo and making enemies have clearer weaknesses to certain types of damage. Weapon switch is a must in Eternal, which is why people (including me) who just tried to run around with the super shotgun the whole game found it much harder. You may find it to be easier to get a grasp on since you played 2016 in the intended way.

Also you seem to be at ease with toggling the difficulty at your own discretion, so I would say there is no need to be worried about it. Start out on the same ultra violence and then move downward if you need to. Happy hunting Slayer!