After having studied the language for about 2 years (not at an entirely invested, about a high ILR level 1), I still cannot get the hang of using the correct declension in all the cases. I understand that it can change depending on the article (or lack thereof) in front of it, but remembering it correctly is still a massive challenge for me. Am I just having my own personal trouble or is this part of the language that is difficult for most non-native speakers to learn? If so, are there techniques people use or is it just a steep learning curve that is improved upon with time?
It is you, but not in the sense that your learning ability is subpar. Almost certainly it is because your native tongue doesn't feature inflection endings the way German does, and apart from the problems of memorizing the values, you have to cope with the concept of them being there at all. This isn't something that you can sidestep - it comes to you after a certain amount of active practice in a language.
If it's any encouragement, I can assure you that the practice does help, even if you have the impression that you're not progressing at all - you are progressing, and the entire system will eventually click into place. It's just that the automatic retrieval of forms necessary for speaking fluently can't be learned consciously, it really does require a lot of practice.
(As inflection systems go, German is actually quite modest. Classical Latin is a freak show of forms in comparison, and people spoke that easily enough, after enough practice.)
Very probably it is not your difficulty but the fact that German grammars have not found yet a way of presenting the adjective declensions in a reasonable form.
There are tree types:
1 After sein the adjective has no ending: Der Mann ist alt.
2 There is an adj declension with the endings e/en. If you look at such a declension table you see you don't have to learn much. The n is lacking in the three forms of nom sg and in two forms of acc sg.
You use this n-declension after the definite article, which indicates with its ending genus and number, so that the adjective does not need to repeat endings for genus and number.
And you use the n-declension after all article-words that have endings for genus and number, that is with the endings -er/e/es such as dieser, jener, welcher etc.
3 You have an adj declension that has endings for genus and number as in großer/große/es. Here the adjective functions as the definite article. You use this r/s declension wenn there is no definite article before the adjective as in guter Wein. Some forms of ein have dropped the endings -er/es. After these forms you use the r/s declension.
For beginners all this is a bit complicated and one has to study this carefully in a grammar with a lot of examples. And of course, you need the declension tables of the adjectives.
Declension tables have various forms,the cases can have the arrangement nom gen dat acc or nom acc dat gen - the arrangement of genus may be der die das die or der das die die. You have to decide which form is best for you and if necessary you have to rewrite the tables in a form that is convenient for you. You should always use the same form of tables. It is totally confusing if the presentation of the tables changes from grammar to grammar or from website to website. Don't learn tables by heart, write them down actively till you see the tables with your inner eye. Study what you don't have to learn. Try to formulate in one sentence where n is lacking in the n-declension. Study where the endings of the r/s declension are the same as the endings of dieser, and where there are deviations. This is active learning. Learning by heart is ineffective.
There is only one basic declension (of adjectives) that you really have to learn, the declension table of dieser/e/es. The declension of der/die/das is already a variant with minor deviations (it is not d-er, d-e, d-es, but der, die, das). All other adj declensions are simple variants.
1 Acussative is the same as nominative (as in English) with the exception of der and den.
2 The neutre column is a variant of the masculine column, with a single deviation: das.
3 The feminine column has only two forms. The plural column is a variant with one single deviation: den.