I just finished a conversion of return to white plume mountain to publish on the DM's Guild, and have also been converting a bunch of old planescape adventures for my current game too. I really second taking existing monsters and just feature swapping them - I do it all via homebrew in dndbeyond and it makes it pretty easy to run too (assuming you have access to a lot of content on dndbeyond). I spent a lot of time on the boss monsters, but it still mostly amounted to taking an existing monster (say an efreet), slapping a spell list from archmage on it, and tweaking that list. Then I added effects from the special weapon they were using, plus stole legendary actions from other creatures. It was some work, but it worked as well as I could have hoped - but its only worth putting that effort in for the encounters you really want to stand out. The other thing I would also agree with is dont worry TOO much about encounter balance after level three - I tend to just pay fast and loose as I run it and add/remove monsters in play. I try to err on the side of easy since its a lot easier to add monsters than take them away. And you always have to be careful at first level, characters are very squishy. The Monster Manual Expanded books on DMs guild are also awesome for giving you buffed up version of existing monsters, which is super useful in conversions. Lastly, I just use DC x + party level for DCs, where X is 6 (easy) 8 (medium) 10 (hard) 12 (very hard). I find it works well enough as a short hand that its not worth putting deeper thought into it. There is just a ton of base content in the game now that is easily plug and play or reskinnable so that converting adventures is pretty easy and actually fairly enjoyable in my opinion.
