Is there a way in Blender to achieve some CAD commands like extend lines to the intersection (or nearest) point, or trim/extend to plane?

Specifically for the commands you mention there exists a CAD add-on called mesh tinyCAD, allows you to easily do the following things (this is not an exhaustive list):

• VTX (like _extend):
• automatically extend edges towards their projected intersection
• generate a vertex at the intersection of two edges ("weld")
• V2X:
• generate an unattached vertex at the projected intersection of two edges
• XALL:
• intersects all selected edges.
• BIX:
• generates the bisector of two selected edges
• CCEN:
• given any three selected vertices (in one object), this can recreate a circular set of vertices of any number and scale.

For questions about the add-on or ideas use the project's issue tracker

For _fillet you can use the bevel command: CtrlShiftB With 1 vertex selected

• It starts out as a bevel but you can increase the number of segments to turns it into a fillet.