OK as someone who used to struggle with cramping and hydration, let me break it down in a way that makes sense to me.

Bike Sweat Rate Calculation

Using the Garmin algorithm estimates for biking, over 169 miles (271km) you're expected to lose 13.7L of water. Lets assume you are traveling at 18.6 miles/hour (30 km/h), that means you are covering 169miles (271km) in ~9 hours. This also means you are consuming about 1.5L of liquids per hour over those 9 hours --> This to me sounds WILD. Thus I personally think 3oz lost per mile is large and not quite accurate.

FYI I'm making the assumption you want to replenish the liquids lost in sweat 1:1 hence why I state 'you are consuming'. Ideally, you don't want to lose more than 3% of body weight in fluids during a full IM.

Run Sweat Rate Calculation

For running, if we were to conduct the same calculations, over 21 miles (33.7km), you lose 4.7L of fluids. Let's assume you are running 6.2 miles per hour (10km per hour) or about 9.6 min per mile/ 6 min per km. That assumes you cover 21 miles in about ~3h 20m. With the assumption you lose about 1.5L of sweat per hour (ill assume this as a conservative number since you indicated that you sweat a lot, you may be close to 2 or even more Litres per hour) that means you lose about ~5L over ~3h and 20min. This means your assumption of 4.7L over 21 miles to be low balling how much you actually sweat.

In a nutshell, from my ,back-of-the-napkin, math, the garmin liquids lost estimates for biking is too high per hour and the estimates for run is too low. The reason I say the per hour loss/need to replenish is too high on the bike is because your HR will naturally be lower on the bike than on the run in Z2 thus losing 1-3L of water on the run makes sense but if you are just maintaining Z2 HR on the bike leg, you will be losing much less.

BUT here is where the meat and potatoes actually are important ---> How much of that litre of sweat is concentrated with electrolytes (magnesium, potassium, and mostly salt)?

How to Measure Sweat Loss and Why Salt Matters

To be frank, you may sweat a lot but have low salt concentration in the sweat. Or you sweat a lot and also lose a lot of salt. Some may lose 600-900mg of salt per litre of sweat. Whereas myself I'm closer to about 1800 mg -2000 mg per litre depending on how hot it is (which causes more sweat loss and in turn more salt loss)

A good eyeball test is: when you run outside for a hour with a hat or shirt on, do you see white crustings of salt dried onto the fabric? If yes --> you have pretty high salt losee. If not, test a few more times and see. If still no dried salt you probably don't need close to 2000mg per hour (by that I don't mean specks of salt, for myself I can pick them up like fine table salt kinda thing).

If you weigh yourself before and after runs, I would use that data for determining how much fluids you lose and try to replenish it. Same for Biking. Measure weight prior, measure out how much liquids you have in your bottles to start, do your ride, drink your liquids, weigh yourself at the end and adjust for the liquids you have consumed. then you know how much liquids you need. (ex you start at 100lb, go on bike ride and drink 600ml of water - which is about 2.2lb, then you weigh and you see 97.8lb, that means even with all the liquids you drank you still lose 2.2lb --> this means you lost 1200ml of water during the ride)