For me I think it comes down to how many you are starting with the same bye. Being best 18 and with the introduction of the flex position, and being so early in the season that we should have the majority of our 31 players (byes excluded) playing means I think it's more manageable than people realise.
If you field 23, plus have a loop on 3 lines, you have 26 scores to get 18. Even allowing for the wrong players to be benched to take a good E score, you should be more than fine as by doing so that E score likely goes into your best 18, meaning you have more changes to drop poor scores.
I actually think a bye in round 2 is optimal as we should have the best starting 31 between those we picked who were named in round 1, and any correction trades. The idea that you lose X points because you are fielding a rookie over a premium isn't exactly accurate, because you have so many chances to make your 18th score better vs someone who has an extra premium on field but potentially different rookies or premiums.
In a vacuum, premium over rookie will likely score more points, but I don't think the difference is season defining. If you had 2-3 in that round 2 bye it might be different, but if it's just Flanders then I think it's easily absorbable.
If you field 23, plus have a loop on 3 lines, you have 26 scores to get 18. Even allowing for the wrong players to be benched to take a good E score, you should be more than fine as by doing so that E score likely goes into your best 18, meaning you have more changes to drop poor scores.
I actually think a bye in round 2 is optimal as we should have the best starting 31 between those we picked who were named in round 1, and any correction trades. The idea that you lose X points because you are fielding a rookie over a premium isn't exactly accurate, because you have so many chances to make your 18th score better vs someone who has an extra premium on field but potentially different rookies or premiums.
In a vacuum, premium over rookie will likely score more points, but I don't think the difference is season defining. If you had 2-3 in that round 2 bye it might be different, but if it's just Flanders then I think it's easily absorbable.
I know for a fact that I ignored the byes last season and it cost me at least 60 points in every bye that I had a player missing. Pretty sure the best score I had as the 18th was a 61 from memory with a couple of 50s and, luckily, a 40 something in one of the rounds that I didn't have to take that 19th score as I hadn't picked any premiums from that bye.
The simple fact is that you're playing your 19th best player while everyone else is fielding a premium in that spot. So it's ~110+ points against ~60. You could get incredibly lucky and have 7 odd rookies that you field all score 100+ but it's insanely unlikely and you should be planning on a 60 type score, for Flanders that's 60 points lost if he plays to his price.
The full rosters is actually a negative as it means the only guy you can loop is that missing premium, so at most 2 lines. Later on would actually be better where you're more likely to have a couple of dead rookies floating around.
There are reasons to pick him still. The obvious is that you think he's got 10 points of upside, he's Dane Swan like as a player so it's definitely possible that he does improve further, I don't think 130 is impossible for him. You might also think that none of the other options are better. The only other premium backs to pick from are Sheezel, Ryan, McGovern, Sinclair, Clark and NWM if I cut off at 100. I can make a case against all of them that has them being 10 points overpriced or unlikely to improve and, unfortunately, we really do have to pick a certain amount of backs still so it's not as simple as just spend that money elsewhere. You can also only realistically pick 2 of Clark, Ryan, Sinclair and NWM without basically just kicking the problem to their bye with interest given a lot of other players from those teams are relevant at other positions.
Personally, I like Stewart and Holmes more if going for a premium level defender with the bye as I think their 10+ improvement scenario is stronger than Flanders. Albeit Flanders does have a lot better premium certainty to me. Even then, I'd probably prefer as an early upgrade target post bye barring enormous r0 scores that are going to kill the value.
Mills is a different story given he's potentially 50 points underpriced on his best.