Настрой яркость на планшете, иначе читать будет неприятно.

Настрой is the informal singular imperative (ты-form) of the perfective verb настроить (to set/adjust/configure). In everyday speech it’s a natural command meaning Adjust it (and finish the adjustment).

• Настрой (perfective imperative) = make the adjustment (as a completed action), often sounding like do it now / get it done.
• Настраивай (imperfective imperative) = be adjusting / adjust (process-focused, repeated, or ongoing).
  In this context (one specific brightness change), Настрой is the most idiomatic.

Use the plural/formal imperative:

• Настройте яркость на планшете, иначе читать будет неприятно.
  Настройте works for you (formal) or you all.

Яркость is the direct object of настроить, so it’s in the accusative case. For inanimate feminine nouns like яркость, accusative = nominative in form, so it stays яркость.

With devices, Russian often uses на to mean on the device / on its screen/interface:

• на планшете, на телефоне, на компьютере
  в планшете usually means inside the tablet (physically), which isn’t intended here.

иначе means otherwise / or else. It introduces a consequence if the first instruction isn’t followed. It’s neutral-common, not overly threatening; context decides how strict it sounds.

Because иначе introduces a second clause with a result/contrast (otherwise…). In Russian, two clauses like this are typically separated by a comma:

• Сделай X, иначе будет Y.

This is an impersonal construction: (it) will be unpleasant to read. Russian often uses this pattern:

• Инфинитив + будет + (category word like неприятно/трудно/удобно)
  You can add the person in the dative if you want:
• Тебе будет неприятно читать. = It will be unpleasant for you to read.
  Without тебе, it sounds more general.

будет is the future tense of быть (to be), used to form the future in sentences like:

• будет неприятно = will be unpleasant
  So читать будет неприятно = reading will be unpleasant / it will be unpleasant to read (especially under those conditions).

Here неприятно is a predicative word (often treated like an adverb in form) used in impersonal sentences:

• Мне неприятно. / Было неприятно. / Будет неприятно.
  It expresses a state/assessment: unpleasant.

After words like неприятно, трудно, легко, удобно, полезно, Russian commonly uses the infinitive to describe an activity in general:

• читать неприятно = it’s unpleasant to read
  No subject is needed; the action itself is evaluated.

Yes, both are natural:

• иначе читать будет неприятно (focuses a bit more on reading)
• иначе будет неприятно читать (slightly more neutral/flowing)
  Russian word order is flexible; emphasis shifts, but the core meaning stays the same.