If you're cooking forma along side a new frame you have, at minimum, 3 additional polarities you can add when the frame is done. The way this game is structured the benefits of planning for the future far and away outweigh the consequences of not so we'll say that OP is able to resist the urge to forma Amprex 3 times while waiting for a new frame to pop out.
Exilus is not all together necessary unless you need a "high level power drift" build or whatever so you can make due without it for a while at least. A potato is another story, but they aren't the hardest thing to come by especially if you do sorties every day and pay attention to alerts/grind out nightwave (which I personally have no issues with but some ppl have mad issues with it I guess). Potatos are just way easier to get these days.
Not trying to argue anything but my point here which is to piggy back off of what you've already said. I can't deny that I'm an Oberon main before anything else. Maining a frame is a great qay to build up the necessary materials to build out a far more robust arsenel.
That said, there are plenty of ways to configure a build for every frame. A potato'd, 4 polarity, exilus adapted Saryn is going to pump out more dps than an unmolested rank 30 hydroid for sure. That doesn't mean you can't slap on some power efficiency and still pump out some cc or be a bonus loot man while Nekros desecrates yoyr victims.
A rank 29 un-mastered Inaros on the other hand, is also going to be nigh invincible just like a fuly built and polarized Inaros.