if subscriptions significantly impacted on impact factor, it is worthless to be specific in subject specific journal. For example, better to target horticulture journal rather viticulture or pomology.
Well it depends on your aims. Do you want your research to reach a target audience where most will read it or do you want it to reach a broad audience where only some will read it. Impact factor is only one metric for the quality of a journal. It is a measure of the average number of citations to recent articles published in that journal. It is known to have a number of problems such as being susceptible to single article citations driving up a journals result for a few years and also the one you highlight - journals with a large readership have more chance of having the articles cited. I suggest looking at a range of metrics in making your decision about which journal you should target. Try SNIPs (source normalised impact per paper) as they are weighted by citations based on the total number of citations in a subject field, getting around the broader readership issue. You could also try SJR (SCImago Journal Rank) to help inform your decision.
Read the scope of the journal before submission whether your research aims is in line with the journal. You need to subscribe to different journals at different times to meet your particular needs. However closed access journals have a better reputation