"I am thinking that it is bad."
Translation:Táim ag ceapadh go bhfuil sé go dona.
This is a really subtle point but I believe the correct sentence should be Táim á cheapadh go bhfuil sé go dona featuring a proleptic á. Since the object of the verbal noun here is a subordinate clause and not a noun that can be put in the genitive, we have to use Táim á cheapadh go... which can be literally parsed as "I'm thinking it, that..." instead of Táim ag ceapadh. An example of this structure can be seen in an EID example here. While I'm reasonably confident what I've written above is true for Munster Irish, I'm much less familiar with the other two dialects so I'm not sure if what I've written applies to them as well.
There is merit to your suggestion, but it should be pointed out that the EID example is for a translation of "I take it that", where the "it" is the object of the verb in English, (not the subordinate clause) and at a stretch, that object would indeed require á in Irish (despite the fact that ag ceapadh is not in any way a direct translation of take). The EID has a number of counterexamples, where the subordinate clause is not treated as the object of the verbal noun.
"You are Mr. Walsh, I presume?" - tá mé ag ceapadh gur tú an Breatnach
"I imagine them to be fairly rich" - tá mé ag ceapadh go bhfuil siad leathshaibhir
"I have been thinking that . . ." - bhí mé ag ceapadh go . . .
"I have an idea that . . ." - tá mé ag ceapadh go . . .
Thank you for your reply. Looking into the matter further it seems my suspicion was correct and the difference is primarily a dialectical one. I suspect this is the reason our examples disagree, further evidenced by the use of the Munster-favored form táim in my lone example vs the analytic forms preferred by the other dialects in yours. I don't think that the presence or absence of a direct object before the subordinate clause in English has any bearing on whether á or ag is preferred in Irish.
I retract my complaint, the sentence here is totally correct. Though I'd still prefer that Táim á cheapadh go... also be accepted.
It does give ag machnamh in the pop -up as an alternative and the meaning of machnamh, used for meditation in the section on religion, is more like your description of smaoineamh.
If the subject of thought was abstract, as in a moral question perhaps in that context smaoineamh and machnamh could work?
For the first time I looked up ceap and saw it's meanings as some material like a block, or something that can be fashioned into an article, and could get how from that you could abstract a sense of inventing, or thinking in a more instrumental way to achieve a goal.