to a conclusion, tell us what your expectation of the end-result is.
Welcome to high definition video. When high quality is needed, lots of space (whether storage media or bandwidth) is required. Compressing the video file can make it use less space, but does so by discarding video data. When video data is discarded, video quality can suffer. Generally, keeping the original video copied from the camcorder, will provide the "best available quality" downstream.
If you want to store the video from the camcorder onto the single layer DVD blanks to archive the video, as you have found, you will use lots of discs. Even double layer discs store only 8.5 gig of data, so increments improvement. Is there a particular reason why you would expect relatively current technology (high definition consumer camcorder) to get along well with (arguably) 20+ year-old technology (DVD blanks)?
Please keep in mind that the parallel technology seems to have been skipped... Blue Ray.
Single layer and double layer discs were designed to store 120 minutes or 240 minutes, respectively, of standard definition video in a very specialized "VOB" format. When HDTV came along and high definition video would not fit, Blue Ray and HD video optical disc technologies came along. HD discs died in favor of Blue Ray. Blue Ray blanks come in 25 gig and 50 gig sizes. Several companies make burners for computers (I like LaCie).
When you burn the MP4 files directly from the camcorder to the DVD, they will not playback on a regular DVD player. For that the video files need to be transcoded from whatever they are to VOB. This makes them standard definition video. That means up to 120 minutes or 240 minutes of standard definition video can end up on the single or double layer disc. When you burn MP4 files to a Blue Ray disc, they will not playback on a regular Blue Ray machine.
If we assume you don't want to playback on a DVD or Blue Ray player and are looking to archive the video for long-term storage at "best possible quality", then you need to update the current use of 20 year-old DVD storage to current Blue Ray optical storage. But there are options - none are "inexpensive".
Assuming you have the various parts, you can see what happens to video when it is compressed. It has to do with screen size... Take one of the video files and using HandBrake, MPEG Streamclip or any number of other transcoders, make three versions that will result in small medium and large file sizes. All three can playback on a small screen like a smartphone and look great. But getting the large file on the small playback method is a waste of limited memory storage on the smartphone. Play the large file on a large screen (42 inch?) HDTV by connecting the computer to the HDTV using HDMI or XGA connection between the computer and HTDV. Should look great... now play the small file on the HDTV... The medium file may be "good enough" for both, but will not be "best available - and if the expectation is to playback on an even larger screen, then , well, we just saw what happens when the screen gets larger and video data has been discarded because of compression (and all three files are still considered "high definition video").
Use external hard drives connected to the computer... but not just any drives or enclosures... I invested in a 4-bay external hard drive enclosure that also provides various RAID capabilities. I use RAID 1. This configuration pairs 2 drives so they have the same data (I copy from my computer once, the RAID1 firmware in the enclosure writes the data to both discs). Because these external drives are electromechanical, they have a "mean time between failure". The drives will fail - but not at the same time. When one fails, it is replaced with a new drive and the RAID1 firmware copies the data from the "good" drive to the replacement. I currently use 4TB drives, in the next round will move to 6TB drives).
Use a cloud-based storage service. This assumes you have a fairly robust internet connection for upload throughput.
So... for archiving important video, you can either spend time (and money on single layer or double layer blanks - and splitting up really large files) or you can spend money (update to Blue Ray, use larger hard drives, use clue-based storage services)... or a combination of these.