I can understand the stressful nature of your dilemma with having to practice piano skills on top of doing arrangements and composition. Personally, before I began to orchestrate full compositions, started with using MuseScore user's compositions and arranging ones that I felt were done poorly or could use improvement or that I noticed places where this or that would've been a better choice to happen in a given section. This jump started a lot of lost knowledge from the time I studied music theory and got the ball rolling for me to actually compose something (whch I'd never done before. This was the results of that (My Musescore Profile. See the Spotlights) - Especially the long arctic midnight. It's a bit long but it's the first full symphony orchestral piece I created. 2 movements in 11/8 time, modulating between 6/8, 12/8 and 4/4 as well. Took me 6 months to do that one.
And as far as the use of Spitfire, you can use them in MuseScore's mixer. Musescore will automatically detect usable VST plugins on your device. Nothing to do further at all. And via MuseHub, you can install MuseFX which comes with a good range of audio effects plugins that are integral to the program itself.
Just don't waste your money on the MuseHub sound packages.. they're mediocre at best in my opinion. MuseSounds out-performs them in terms of their interactions with the sheet music during playback. The sounds themselves are A+. Their ability to react to dynamics and stay in tune however, not even close to an F rating in my opinion... Trumpet sounds are pitchy, and other brass sounds don't follow dynamics like they're supposed to. Strings are also pitchy and cannot perform any sound flags like they're supposed to, the rhythms aren't ever played back correctly etc.... way too many bugs imo. Spitfire, some of their sounds are better than others. Especially if you're using LABS free edition. I pay for LABS+ and I gotta say, it's an improvement. However, some of the MuseSounds are still top tier compared to anything from LABS. So I just use them in tandem for anything I create now. Mostly I use LABS+ for the Piano playback sounds they offer. Much more realistic sounding. Their Choir sounds are awesome as well, and their Intimate Brass is useful for Trombone and French Horn playback which MuseSounds lack quality in those areas. There isn't much in the way of woodwinds, and the ones they do have don't sound nearly as polished as Muse does, so I don't even bother with them.
Basically, nowadays and VST3 playback options out there are currently lacking in their ability to interact well with notation software, on the general level. I don't use DAWs so I can't really say much to their quality in that aspect, however, I have seen some works out there using LogicPRO, FLStudio, and other DAWs using these VSTs and they come out absolutely awesome. I'm just not privy to that software. I'm much more accustomed to reading and notating sheet music since it's something I've known from a time before I knew anything about DAWs or even Notation software.