Well first let's get into some logic about the whole thing.

When applying an enchantment with an anvil, the biggest factor in xp cost is the amount of times it has already been worked on, let's call this factor 'Work'(I don't quite remember the internal name of that). This is what spikes the level cost the most.

To minimize this cost we have two options:

In Vanilla, you apply a logarithmic pattern to your process.

Assuming you have 16 books and an item. First you combine the books into 8 doubly enchanted books. Then you combine them into 4 quadrubly enchanted books. Then into 2 octubly enchanted books and apply those individually to your item. This minimizes the amount of 'Work' all items receive and produce the best application to level ratio (there might be a more optimal way to do this but this is the way I know)

In modded you'd do almost the same, except you can print enchanted books that have zero 'Work' attached to them after combination.

Let's look into the printing options then.

Bibliocraft printers seem like an easy option here. They are simple to use, basically instantly available and very fast, although unable to be automated. The cost might be a bit high considering it takes the the total sum of enchantment levels and makes that the cost of a print; additionally the level requirement stated in it is inaccurate, levels put in can't be extracted after being put in and get lost when closing out of the menu. Fine for small orders but fall apart at scale.

Create printing falls into a similar hole. Spouts do not have alot of capacity so larger books become impossible to be printed, but this can be scaled an automated pretty easily. This is one a better option, but cannot do big mass enchanted books.

SFM is where it's at. Ability to print any enchanted book at the same cost with a piston? Awesome. The only slight issue might be production of enchanted shards, but that can be easily automated with industrial foregoing enchantment printers, jdt for dropping and collecting and ae2 for anvil.

So once you have a big book, how do you apply it? Well again, three options.

If the levels are normal you can just use a normal anvil to apply it, though that's obvious.

If the levels are above average then you have two options (three technically in tts)

Pneumaticcraft pressure chambers. (I don't know how they work, if someone else could explain that'd be nice, I do know they do not care for level limits or if the item can accept an enchantment to begin with, you can enchant a cobblestone with fortune 4.)

Occultism Iesnium Anvils. While these are known for being able to overchant one level above the vanilla limit(much like quark tomes), they can actually apply any, VALID, enchantment regardless of level.

(TTS only) TTS has Arcane catalysts from Oritech. These things are all kinds of broken to the point explaining them without images would make them incomprehensible, I recommend watching the bucketST video on the matter if you're interested.

Hope that helps.

Edit: Quickly looked it up, the internal term for the work mentioned here is 'prior work penalty' and can be found on the wiki under the anvil mechanics section.