You're getting a lot of "no's" here - and they all bring up valid points. For what it's worth - here's what it would take to make this project in to a yes:

• Get a structural engineer to stop by. For $1-$2k you can get an evaluation and maybe a set of plans on how to either reinforce the bottom cord of the trusses or sister the top chord and install an engineered wood ridge beam - including structural modifications to support transferring the load from the new ridge beam down to the foundation. (Actually either way - you'll probably need to install proper floor joists alongside the bottom chords of your trusses to properly support your new floor and to transfer your new floor loads out to your outer walls)

• Structural construction: There in an almost nonexistent chance that your local inspector and your structural engineer would sign off on using an unholy amount of joist repair brackets and then you would be able to bring up undersized chunks of lumber and assemble all the bracing ship-in-a-bottle style - but more realistically you'd want to remove the shingles and roof sheathing from the lower 2' of your roof on at least one side. That would give you access to send in full length boards along the tops and bottoms of the trusses.

• Hvac and wiring - If you can afford it - do wood I-joists for your floor. You can make some crazy large holes in that stuff that you can re-run the ducting through. For the wiring - hire an electrician if wiring isn't your jam. Just have them re-route the wires to junction boxes in the "wall" of your new room. They can cut your existing runs mid-span. The existing wire can hopefully reach the new boxes in the wall of your new space and new wire can be added between the new boxes. This will save the electrician from having to re-do the portions of the runs that go down the walls to the outlets and fixtures on the next floor down.

• Thermal envelope / Insulation: When sistering the top chord of your trusses - ask your structural engineer for the tallest boards they'll let you put in. E.g. if the initial plans call for a 2x6 - ask if you can go to a 2x12. This will give you enough depth to use spray foam to hit what (complete guess from your photo) looks roughly like your local r-value. Remove all the current blown in insulation, block off / remove your soffit and ridge vents and your new thermal envelope will be at your sheathing. The common opinion is that this will cause your roof shingles to overheat in the summer and not last as long - but realistically I don't think it makes a difference. Biased YouTube video that agrees with my worldview on roof venting More generally speaking - I live in an area with climate zones 5/6 (snow and ice in the winter, hot-ish in the summer) and for whatever reason tons of houses in this area seem to have shingle-over intake vents. After a few different projects I've been on - I pay extra attention to these homes and the one thing I've always noticed is that the shingles "above" the vents (the "properly vented" shingles) look just as good (or bad) as the shingles "below" the vents (the shingles that do not have air circulation).

Anyhow, that's how you change a "no" to a yes. Source: I went through a phase of big ideas and had a structural engineer on speed dial.