I use Lightroom and routinely save image files in camera as both RAW and JPG simultaneously (I no longer shoot much film, only digital, but bear with me). As I understand it there should be no difference between JPGs created in camera and JPGs created by scanners.
I can say that my Lightroom (a standalone version - 5.6 I think) works perfectly well with a range of file types - TIFF, JPG and various RAW types.
But in general, I prefer to use RAW files over JPG and do so almost exclusively (or if I did not have that option, I would use TIFF files for processing in Lightroom). The reason of course is that JPG algorithms throw away data when they are create JPG files. This is done in the interest of having smaller files, but the cost is sometimes noticeably poorer image quality - especially in the shadows and highlights but also overall. For example if I have blown highlights a RAW file will almost always allow me to pull some image back out of the blown area. While a JPG file almost never will as some, or most, or all of that data has been thrown away by the algorithm that created it. This applies to JPGs created in a camera and to JPGs created in a scanner.
These days, computer hard drive storage is cheap and I think there is no reason why I would use JPGs over TIFF or RAW files. The only reason I create both JPG and RAW simultaneously in camera is that my computer does not have the necessary CODECs for some recent RAW files from newer cameras. So they do not render under Windows so instead I use the JPG files only as a kind of thumbnail to whats in the accompanying RAW file, but its the RAW file that really interests me. Once I import these into Lightroom this is no longer a problem in any event as LR handles both file types. In fact what I normally do is to elect for Lightroom to convert any RAW files I import, into DNG files within Lightroom. This means I do not have to worry about whether they will be compatible in future as DNG is about the most universal RAW format around.
You asked about converting from JPG to TIFF. I do not think this helps you much if at all. Once a JPG has been created, some data has been lost. Converting that JPG to TIFF will not put data back into the image once it has gone.
Short answer to your question - save your scanning output as TIFF files not JPG as you will get better end results most times. Having said that sometimes the improvement is only small but it is there. The other reason is that if you save as JPG files some decisions will be made for you by the scanning software. Saving as TIFF will also buy you better flexibility in this respect too as otherwise you are more or less stuck with what the JPG algorithm has given you.
BTW I prefer my standalone LR as I am not keen to pay a monthly fee for software but then again that is me. If I were doing hundreds or thousands of files per month and doing that regularly month after month, or if I only wanted to subscribe for a couple of months to complete a scanning project maybe an subscription version would be a better value proposition for me. The other reason is that I am not sure if the latter works with plugins and I like using plugins running under LR.