Need direction - Bosch Singular 5200

Around 65' of baseboard you can run on each loop …. 92' is too much on a series system …You can parallel your system in half with 1" supply and return … Add purge valves to remove air on each loop…

Panel radiators and the outdoor sensor works well with your boiler …

The key question on the 90 feet. And I'm not clear on the piping. Does the water flow from the boiler to one section of 30 feet. them on to the next one, then on to the last one, and then back to the boiler? Or does the water flow from the boiler individually to the beginning of each 30 foot section, then return individually to the boiler? The first arrangement is called "serial" piping, and the second "parallel".

Now if it's serial piping, what happens is that the water temperature drops as it goes through each section, as the heat is given off by the radiator. So the first section gets nice hot water, the next one sort of medium, and by the time the water has made it to the last section in line it's actually pretty cool. And that last section just won't heat.

"Hybrid" would be a kind word. Is there any way you can rearrange the piping so that each section is fed individually from a manifold of some kind — doesn't have to be right at the boiler — and each returns individually to another manifold, and then the whole lot returns to the boiler together?

You may not need a three speed but is nice to have .. A parallel baseboard system needs to be balanced . It will take time and adjustments to even it out . Once set it is left alone . Until someone down the road plays with it :) … Not my choice of heating system , just adding…

The output of fin tube baseboard is poor at condensing temps, you usually need around 150-180 degree water to get decent output(the manufacturer of the baseboard will have a table in the manual). You can do it with lower water temps but you need a whole lot of baseboard. Panel radiators or radiant are much better for low temps. That being said the efficiency gain from running lower temps likely doesn't justify the cost of changing the emitters.

3/4" pex is more like 5/8" copper so you likely will need to use 1" pex per loop to get enough flow per circuit.

You can use a single circulator but you will need to have a balancing valve (either a ball or globe valve, doesn't need to be specifically sold as a balancing valve but can't be a gate valve) in each loop so you can balance them.

If you use multiple circulators put one on each loop, do not pump one in to the other. The multiple circulators has the advantage that when you decide you can't get it balanced because the heat loss of the different parts of the house change in different ways under different conditions, you can get a zone controller and put flow checks in the circulators and make it 3 zones.