Good instincts trying to compare the stack and reach taking into account the stem, stem angle, etc. I’d start by taking that to the next level and either busting out your high school trigonometry skills or finding an online geometry calculator that will give you handlebar X and Y (or stack and reach at the centre of the handlebar clamping area) so you can really compare apples to apples. A true calculation should include head angle, stem angle, stem length, height of the mandatory headset cover/spacers, optional spacers, and height of the stem clamp. There are a few sites I’ve seen out there that will do that but I can’t remember which ones, sorry (I’m tracking mine in a spreadsheet I wrote).
Also remember that different handlebars have different reaches, and different shifters have different reaches as well, especially if you’ve got different brands on different bikes. I don’t factor those numbers into my math because they can change pretty drastically depending where you mount the shifters on the bars and on whether the bars are rolled more forward or back from one bike to the next. Nearly impossible to quantify unless you’re using identical parts and measuring angles, so I don’t bother. However, I do think it’s helpful to know if the reach on one set of bars and/or shifters is 5-10mm different than the other as that can affect things - it’s just you have to include some guesswork as well as the numbers.
If you want to know if your saddle position is truly identical across all bikes you’ll need to measure not just saddle height but also saddle setback relative to the bottom bracket. There are fancy tools for this but you can get a pretty decent DIY measurement with a long builder’s level and a tape measure. Make sure the bike is level on the ground (or if you want to do it in a workstand just make sure the centre of your wheel hubs are level with each other), then use the level to get a vertical line down from the tip of your saddle and use the tape measure to see how far that line is behind the centre of your BB spindle (on Shimano this is easier on the non drive side because of the open crank end).
Maybe don’t mess with saddle position if you’ve already been professionally fitted on both bikes as there are lots of reasons you might not want the same saddle position relative to BB from one bike to the next, but if it’s all a DIY job then it’s usually a pretty good starting place to replicate this setup across all bikes.
Then you’re finally ready to compare your numbers (and to look at how you’d likely end up setting up a new bike based on an online geo chart).
FWIW here’s my N=1:
Compared to my road race bike my racey gravel bike has handlebar XY at 13mm higher stack and 21mm shorter reach and it’s not long enough for me. Stem is already as long as I’m willing to go on that bike and it’s nearly 4 years old so I’m looking to replace it for something where I can get more like +13 and -10. In the meantime I’ve had to increase the saddle setback to get a similar length but I’m aware that it compromises my pedaling a bit.
I have another gravel bike that’s a bit more adventure and ultra-endurance focused and compared to my road race bike the bar XY is 25mm higher stack and 21mm shorter reach, and that one doesn’t feel too short despite being the same length as the ‘cramped’ racier gravel bike.
All three bikes have similar enough handlebars and shifters that I consider them broadly equal. So for me it seems like I can raise my stack and shorten my reach at roughly a 1:1 ratio (for at least the first 25-30mm). I’m sure I’ve read somewhere that it’s more commonly a 1:2 ratio but I can’t recall where that was or even which way round was the 1 and which was the 2.
I know that’s not a direct answer to your questions but it’s how I would go about making real comparisons if I was thinking about committing to a bike sight unseen, which is effectively what you’re looking at with the Otso.