Old gamer here too. I recently finished the main story and two expansions for the first time and have started a NG+ run. Here are some pointers that should help you along the way.
You can't just button-mash until the bad beast/guy is dead like in many other RPGs. It'll take a week + to get used to the controls and the flow of combat. Use the difficulty slider so you can get a feel for the game and flow of combat first. Then after maybe getting to around halfway through Velen (or however long it takes for you to get used to combat), restart on the normal difficulty level. I currently play on B&BB difficulty I dont really enjoy Death March.
Definitely watch/read tutorials and the instruction manual so you at least know how to do everything. For my first 10 days of play I never thought to combine mutagens. By 10th level and beyond, you will sometimes spend minutes in your alchemy, inventory, and character menus and with armors/blacksmiths dismantling and creating stuff.
Don't worry about 'optimal' builds. In Velen (and further on) you can get a potion that resets all your ability points so you can try out different builds. It costs 1k but you should be swimming in coin by the time you can get one.
I've found that dismantling most things isn't worth it since you have to pay to dismantle. I get so many runestones and components that the only time I do dismantle is when I have that one specific component that I need to create the griffin sword (ex.) and I happen to have something I can take apart to get it. I never take apart swords or armor to get runestones back. I just sell them because I have 5 more of that exact same runestone. Crafting and alchemy components don't weigh anything so collect all and just sell the excess. I usually sell some once I get above twenty for the total number of a specific component. By the time I'm level 15 I have over 15k in crowns (gold) By the time I'm level 20 I have 50k and have stopped searching every box/bag/whatever unless it's directly on my way.
Parry is so much fun. Before using it regularly I was like, "Let me just roll to the archers to take care of them." Then I figured out two things. First, parry works in EVERY direction! As long as you're holding parry you will block standard attacks from ALL sides - not just the direction you're facing! Now I just hold parry all the time with humans and sometimes with monsters (some monster/beast attacks and all human attacks with two-handed weapons will stun you or go through parry. You'll have to learn which ones you can use parry on). Second, spending one point in the 'arrow deflection' ability gets you a really good defensive stance that can deflect arrows, swords, maces, and such from all angles while you move through combat.
Read your beastiary, alchemy, crafting, so that you know creature weaknesses and have an idea of what items, potions, decoctions, oils, etc. you can construct.
On higher difficulty oils and potions/decoctions are practically a necessity. On the 'normal' difficulty they make you superman!
If you are a completionist (like me) you will be above the recommended level for many quests by the time you get to Novigrad. If you are more than 5 levels above the recommended level you only get a pittance in XP for completing that quest. I was able to stay within the recommended level range for most things through Velen by doing only the Velen main quest stories (hunting the witch and the crones) as soon as I arrived and THEN going through all the side quests. However, the Velen side quests get you up to ~16th level. Past many of the Novigrad main quest recommended levels, BUT that's perfectly fine! If you go this route you'll just stay at the very top end of the recommended level for the areas/quests that you are doing.
Use your stash chest in Velen and Novigrad (and other locations) to store Witcher gear you aren't currently using but don't worry about carrying around an extra full set of armor/weapons for specific reasons. Some armor is much better suited to fighting humans and other armor better for monsters. Pay attention to percentage damage reductions for specific types of attacks. Most of the time percentage damage reduction will help you more, even if that armor has a base armor protection that's 10-20% lower than another armor. Also, buy better saddle bags and equip Roach with them ASAP. Better saddle bags = more carry weight.
Mods are really down to personal preference but I'll give you my list.
FCR3 (and compatibility patches), HD Reworked Project, NPC Scabbards, Community Patch (Base and Besserwisser), and World Map Fixes.
I tried playing Witcher 3 without FCR3 and, for some reason, FCR3 feels more like vanilla than vanilla. I love what it does to the game (including all the bug fixes) and can't play without it but, again, to each their own. I looked into Ghost Mode and the Enhanced Edition and don't want to change the game that much, yet. FCR3 was made by one of the original developers of Witcher 3. I like his minimalist changes (but big impact) way of doing things.
I'm sure I have other pointers but can't think of more now. I'll add more as I remember them.