I'm lumping your questions back into one because the answers go together:
If I accept the pre-qualified offer now, is there any significant downside in the short or long term? If I wait a while, assuming my balance/credit ratio is unchanged or even improves during that time, would it still be a good idea to apply for a pre-qualified credit card, given that enough time has passed since my last hard credit check?
There might a downside, but probably not in your situation. If you accept one credit card, the following things are likely to happen:
- Most likely there will be a hard pull which will have a minor negative impact on your credit score. The impact will decrease over time until it falls off in 1-2 years.
- Your AAoA will decrease which will slightly lower your score in the short term, but has the potential advantage of increasing your comparative AAoA when you obtain new credit long term. (Because future new credit will be a proportionally smaller change to the denominator.)
- Assuming you don't increase your spending due to having the new CC, then your ratio will decrease because you will now have more available credit. This will increase your score. (Unless your current ratio is already close to 0%, but you mentioned you have a small consumer loan.)
Every situation is different, but in general, the net effect of the above 3 items for accepting one credit card (IMHO) is that your credit score will slightly increase because the decrease in ratio of your current debt will have a bigger impact on your score than the hard pull and AAoA change. (Except during the time of the hard pull and when the new card reports- which could take a month.)
Now, even if your score would slightly increase, that doesn't necessarily mean you should get the card, and it definitely doesn't mean you should repeat and keep getting more credit. At some point having multiple inquiries and the bigger drop in AAoA will start to have a bigger impact on your score. Furthermore, (outside of the psychological urge to spend when you have more credit), if you have too much available credit, a lender may be hesitant to extend more to you. This would depend on your income too, but the basic line of thinking is: "Based on your income, how much can you afford if you max out all of your available credit?" At some point you hit a limit and they either won't lend to you, or they will ask you to replace existing credit with their own (i.e. cancel some unused credit).
So how should you interpret all of this? In general, I like to have a main CC and one backup CC, so if you only have one card I'd consider getting one more in case you lose your main one or if the number is compromised and you have to disable it until you receive your new card in the mail. Outside of that, I'd ignore the rest of the offers because they aren't going to help that much and they could possibly hurt.
If you decide to get one more card, make sure you pick one that has no annual fee, and if you are shopping, you might as well pick one with good perks. Who knows- maybe you'll find one that is even better than the one you currently have and your current card will become the backup.