I feel u there. 2 tips that got the ball rolling for me with tinkercad. 1st always drag a ruler onto the workplane (I don’t know why it isn’t there by default) and you can change dimensions by clicking on them and entering manually. 2 if you need to select nested parts individually (like if you need to group only 2 parts out of the whole model) you can shift-click to select only the oarts you need. By holding down shift you select multiple parts one at a time. For me these tips made it a tool i use daily. There really isnt a need for anything more robust if all you sre doing is cutting and pasting parts together.