I have a Cast Iron Crown 105,000 BTU boiler. Unfortunately my current setup is bringing the return temp back at 90 degree's. Which is 30 degrees to low. I installed a 3 way thermostatic mixing valve thinking that would solve my problem and ended up creating another problem. After some research it appears i have 2 ways to solve this and I would appreciate some advice. Please see the attachments below.

Where are the attachments?

Really, your HR floor heating system is designed for a 120F return temperature? You must be using primary/secondary plumbing to keep this boiler happy. Is this a new problem on an old system or a new system problem? Who designed and installed this system? What kind of building are you heating and what is your HR floor emitter? We really need a lot more info to sort this out...

I got the attachments to show up. This is what I was told about the shop. 32'x41' Steel Building with an 11' ceiling. The siding is 12" insulated panels that lock together with a tin roof with some batt and blanket insulation. It has a 6" concrete floor with 2" foam panel insulation underneath it and around the sides. the pex is attached to the rebar about 3" down. There is two 1/2 pex lines that are 150ft that run around the building and two 360ft lines that zigzag in the middle.

I would suggest you obtain a copy of John Siegenthaler’s “Modern Hydronic Heating” as it fully explains all the options to address this issue. In addition to the options in your attachments, you could also use a pressure differential bypass valve to keep the boiler happy. You need to ensure that that boiler gets adequate flow rate as well as adequate temperature. 1/2” PEX circuits should not exceed 300’ length.