Self-coached athletes - how do you equate intensity across sports?

Currently I operate on a regular-ish schedule. I do a hard swim session with my tri club on Thursday PMs, and run or bike intervals on a sunday. I try and do a long-ish 2-3 hr thing Wednesday afternoon if feasible. And one gym session per week.

This week I did a tempo bike and VO2 run intervals and I'm pretty cooked today. I'm also not really getting any faster at running but tbf I've been doing that more in recent months so less improvement capacity.

My garmin rates my swims as pretty consistently tempo even though I'm totally dying during them, I guess because it is using my run lactate threshold to assess it. How do you guys avoid doing too much when swimming? I'm at a point where holding 2:00 per 100m all day is easy, but holding 1:55 per 100 for like 400m kills me.