Correct - selecting journals is a complex task and depends on many different factors. You are specifically asking about specialised versus multi-disciplinary. In the first instance, the main question that you should ask yourself is 'how discipline-specific' is your study/article? For instance, I know a few eminent colleagues who work in very 'niche' disciplines where there are only several thousand academics and one or two specific journals. They network with the journal editors at their annual conferences etc and know the journals very well. They are almost guaranteed publication - and are well known and respected in their fields. If they want wider recognition, they have to write purposively for broader multidisciplinary journals which are more competitive and the subject may not be of great interest to other disciplines or readers of those journals.
If you discipline is 'in-between' specific and more general - then it can be a god strategy to target both. It's a bit like if you conduct a study in a specific country and target a national publication but, at the same time, the interest and impact may extend beyond that country and so, with a little adjustment, can also target an international journal.
you may search in doaj https://doaj.org/ by article topic, and look at the journal its been published. Then you may target journals that publishes in the field
Indeed, it is a complex yet crucial task. To go with the specialized journal is quite the preference for proper reviewing of the work. However, multi-disciplinary journals can also be considered for the publication depending on the specificity of your work. There must be match between the subject of the article and the aim and scope of the article otherwise; mismatch became the most common reason for the rejection. Read the aim and scope and then go through the recent published issues.
Though you have been suggested with the journal finder, but I would like to add more in the list:
There are certain tools available for Elsiever and Springer journal, Elsiever journal finder and and Springer journal suggest, you can copy paste the title and abstract, they will streamline certain suitable journals for you. However, you have to decide the worth of your work. You can start with bit high expectations, for eg if you find that the similar quality of work is been published in 3-3.5 impact factor journals than you can start with 3.5-4 IF journals.
The last part of your question is about the multidisciplinary or specialist journal, if you have crisp and novel data go for specialist journal, but if you feel that some or significant information in your article is already published than go for a multidisciplinary journal. Though it tricky, experience will teach you the rest. Good luck
you can also use "Elsevier Journal Finder" or "Springer Journal Suggester". Just write the title and abstract of your manuscript and you will find suitable journals with the impact factor, time of reviewing process, .....etc.