In formal writing you often signal which topic or which angle you're about to discuss. Open with regarding or as regards to name the topic, use in terms of to name the particular aspect you're weighing, and use as far as ... is concerned to frame one specific area of a bigger question.
Pointing to a topic or aspect
regarding /rɪˈɡɑrdɪŋ/ — Regarding your question, we'll need more information before we can decide.
in terms of /ɪn ˈtɜrmz əv/ — In terms of cost, the project is far more expensive than we expected.
as far as ... is concerned /æz fɑr æz ɪz kənˈsɜrnd/ — As far as the budget is concerned, we still have some room to negotiate.
Not ""In terms of that the project is expensive, we should cut features." (followed by a clause, not a noun)" — say ""In terms of cost, the project is expensive." (followed by a noun phrase)". In terms of takes a noun that names an aspect (cost, quality, time), never a full clause. If a 'that' clause follows, the phrase is being misused.
Try it: Which sentence uses 'as far as ... is concerned' with its fixed structure correctly? (As far as the timeline is concerned, we're still on track. / As far as you are concerned the timeline, we're on track. / As far as concerning the timeline, we're on track.) Answer: As far as the timeline is concerned, we're still on track. — The set frame is 'as far as + noun + is concerned', so 'as far as the timeline is concerned' fits perfectly. 'As far as you are concerned the timeline' breaks the structure and drops in the wrong noun, and 'as far as concerning the timeline' jams two different markers together, which isn't English.
Formal ways to say 'about'
In academic and professional writing, plain about often sounds too casual. Reach for concerning, with regard to, or with respect to instead. Each one means roughly "on the subject of" and is followed by a noun or noun phrase, not a clause.
concerning /kənˈsɜrnɪŋ/ — Concerning the matter you raised, I've attached the full report.
with regard to /wɪð rɪˈɡɑrd tu/ — With regard to your request, a response will follow by Friday.
with respect to /wɪð rɪˈspɛkt tu/ — With respect to the proposal, the committee still has reservations.
Not ""About your question, we need more information." (correct grammar, but too informal for a formal report)" — say ""Regarding your question, we need more information." (formal register)". About isn't wrong, it's just informal. In academic or professional writing, swap it for regarding, concerning, or with regard to.
Try it: In a formal email, which opening best replaces the casual 'About the delay, ...'? (With regard to the delay, ... / About regarding the delay, ... / With regard to that we were delayed, ...) Answer: With regard to the delay, ... — 'With regard to the delay' is a formal marker followed by a noun phrase, exactly what the register calls for. 'About regarding the delay' doubles up two markers, and 'With regard to that we were delayed' wrongly follows the marker with a clause instead of a noun.
Try it: Which sentence follows its discourse marker with the noun phrase it requires? (Concerning the funding shortfall, several options remain open. / In terms of the deadline is tight, we should reprioritize. / As regards that the results were mixed, we need another trial.) Answer: Concerning the funding shortfall, several options remain open. — 'Concerning the funding shortfall' correctly takes a noun phrase after the marker. The other two attach a full clause ('the deadline is tight', 'that the results were mixed') to markers that only accept nouns, so both are ungrammatical.