What the OP is looking at is tech diving but on the cheap. Using a second gas mix for decompression is something I wouldn't want to rush into and I would suggest others not do it either.
For extended duration dives beyond NDL, you should be looking at a minimum of a twinset to give suffiicient gas (especially at the depths you are looking at) as well as carrying a minimum of one deco stage tank. Learning how to use the twinset safely (doing shutdowns etc) AND making safe switches to your deco gas while holding station at a specific depth takes quite a bit of skill and training. You also need the ability to problems solve while at depth as you have no direct safe exit route once you rack up deco - you need to either solve the problem there and then or risk an early ascent and possible serious DCS injury or even death.
If deeper & longer dives are in your plan, take an AN/DP course. It will give the skills you need to stay alive and well.
For extended duration dives beyond NDL, you should be looking at a minimum of a twinset to give suffiicient gas (especially at the depths you are looking at) as well as carrying a minimum of one deco stage tank. Learning how to use the twinset safely (doing shutdowns etc) AND making safe switches to your deco gas while holding station at a specific depth takes quite a bit of skill and training. You also need the ability to problems solve while at depth as you have no direct safe exit route once you rack up deco - you need to either solve the problem there and then or risk an early ascent and possible serious DCS injury or even death.
If deeper & longer dives are in your plan, take an AN/DP course. It will give the skills you need to stay alive and well.