As a default this can work - but I prefer the 4e approach where the extra actions can depend on the boss monster. For example each dragon type got a different extra action by character (so the blue lightning struck, the green did a flyby attack, and the white just charged you down) - but while Beholders only got one action they got to eye-ray each PC who didn't spend their entire turn in the anti-magic field on that PC's turn. Then there's the third option where casters and dragons alike get inherently more dangerous with more PCs because they have a lot of AoE attacks that hit everyone.I believe the best way to solve the problem of bosses not having good action economy would be to give bosses extra turns per PC. Half seems like a good number.
All boss monsters should have some way of scaling to larger parties but half as many actions as PCs is far from the only way and is pretty limiting in terms of boss type.