This is just my opinion based upon my personal experiences; I know perfectly well that they will not necessarily align with others' experiences which is fine as I am not here to get into a pissing match.
If your Nikons are not cutting it...
Do consider getting alpha 8x as they will cover the bulk of your current hunting scenarios. Do be aware that you will be sacrificing your ability to find game at longer distances.
Do revisit the viability of 10x for your intended hunting preferences. In some situations they may be too much (ex: for close in work) but still be inadequate for finding game at longer distances. Folks love to say that they have no issue finding game at X number of miles while hand holding 10x. Based upon my personal experience, most game that I find are either bedded or mostly obscured and not perfectly broadside in the open with perfect light. I no longer bring my SLC 10s when I hunt big country for small-ish game (which includes Coues, javelina, aoudad, desert mule deer, etc) as I know I am missing a lot of game and wasting time by constantly grabbing my spotting scope. But at the same time my SLC 10s are too much for short distances (ex: hunting whitetail from a stand or turkey hunting in dense vegetation).
If your Nikons are cutting it (assuming you will eventually cave and use a tripod)...
Scrap the idea of using alpha 12x. They are a compromise and will be less than ideal for glassing longer distances compared to the NL 14s. 12s can be too much for close in work and not enough for longer distances. I am basing this off chasing Coues and javelina for 1 month last year in Mexico and being able to compare the NL 12s to SLC 15s the entire time; dawn to dusk in true rain/shine/sleet/snow conditions. From a pure glassing perspective (on a tripod), there is no reason for me to buy the NL 12s and retire my SLC 15s.
Do consider getting alpha 14x/15x binoculars and build out your optics arsenal. Assuming an appropriate glassing skill level, you'll find significantly more game with them than comparable glass quality 8/10/12 binoculars. I was able to spend 3 weeks this year in Mexico comparing the NL 14s with SLC 15s; dawn to dusk in a variety of weather conditions. For glassing (on a tripod) the biggest advantage of the NL 14s over the SLC 15s is the FOV. This *may* allow you to find game faster. I currently am on the fence on whether to grab a pair of the NL 14s; am unsure if that extra drop of juice is worth the squeeze.
I would consider going with "lesser" optics and save some cash. EL 8.5s are about $2K while the NL 8s will cost about $450-$950 more. EL 10s are about $2K while NL 10s are about $1K more. SLC 15s are $2400 and the NL 14s are about $1100 more. These are "buy now" prices straight from a common web site and do not include any special angles on getting a better price (including calling a RS sponsor and seeing what they can do).
If you are adamant about not using a tripod and want to scan (yes scan and not glass) big country, then look at image stabilizing binoculars such as the Sig 16x or 18x. You will still be sacrificing your ability to find game but you will find more game at distance than relying upon handheld 8x or 10x. This past fall I was hunting blacktail in Oregon. The guide used Sig IS binoculars and was able to find game pretty easily and quickly. But he only was able to find them consistently when looking for the white face. I smoked him on total game found when using my SLC 15s on a tripod. With that said, the Sigs are on my shortlist on optics purchases as they are well suited for use in the truck or side by side.
Do want to apologize for the lengthy response.
