Since you say your comprehension is OK, I would focus on speaking. This is a fundamentally different skill and one that books or individual courses will not help you with. Depending on time, money and local availability, you could try
- Foreign language practice groups - these may be held in local libraries, cultural societies or Universities. Note that they may have moved to Zoom or other virtual format for the time being, or been suspended altogether; in that case, you may still be able to contact the organisers and find someone for
- A conversation partner. A cheap/free arrangement would be to find a native German speaker who wants to practice their English and dedicate half the time to each language, or converse 'cross-language' (you speak to them only in German, they reply only in English, and you each only use your native language for clarifications and corrections). More expensive, but perhaps more efficient, would be
- A professional language teacher. This would probably allow you to specifically request a certain focus (e.g. do you need to be fluent in social conversation or give polished formal presentations in German?) and a teacher may be better at identifying your level and challenging it. I'm sure lots of teachers are now available for remote learning (indeed you could contact someone in Germany) and would be glad for the income given that foreign language schools/summer camps for kids are presumably cancelled.
Your post suggests that there is a specific contingency in your life that has brought this up. If this is work-related, see if you can access any funds for a teacher. In different times I would recommend taking a solo holiday/'full immersion language course' (not even an organised course per se, but you may find families that would host you and commit to some conversation for the price of room and board) in a German-speaking country and try to get out and about as much as possible, but this may not be an option right now.
Keep in mind that Austrian German is a specific dialect of German - it would certainly be understood by all German speakers but also be distinctive. Think whether it matters to you to speak a specific variety of German, or whether you may find it easier to pick up the version you are already most familiar with, in choosing a teacher or conversation partner.
Reading books will be great for vocabulary (especially if you expect to use your German in a specific professional setting and need to acquire the relevant jargon) and listening for comprehension, but the only thing to get you speaking confidently is lots of speaking. I personally wouldn't encourage you to pursue formal GFL courses - by the time you can follow a TV programme with little or no support, you are well beyond what most courses would be able to offer you anyway.
Also, you will have retained much more than you think - especially having spoken the language in your school years, and so presumably having both innate grasp and formal use in your past experience. Good luck!