This is solid advice, but I would argue that the difficulty most pugs run on for leveling is no different than what can be solo'd. In 2017 I started out soloing R3 with 0 reaper points on an arti multiclass build and after the first few weeks of learning curve I rarely died. I did go through a few cakes those first few weeks solo. Reaper has been adjusted and is generally easier now so now I would approach it a bit differently - instead of R3 at level (level 3 quests at level 3) I would run:
Level 1-11: R2 (for the simple reason reaper pay-out is so low the goal is to get it done asap and the reward for pushing skulls isn't much. but R2 is hardly different than R1 so why not. On the other hand if you are breezing through R4 there is no reason no to run R4 - as you gain rxp that will just happen at some point, but starting out R2 is a good goal)
Level 12-29: R4 (generally do-able, but some quests you might want to drop down to R1 or R2 or just run elite)
Legendary: R10 or the highest skull you can in a group, even a pug (Legendary is where most reaper points are accumulated, however, if you really don't want to group you can run R4 solo and still accumulate points reasonably fast).
OP is missing the easiest reaper accumulation by tr'ing at 20 instead of 30 although you accumulate lifes much faster that way. Legendary reapers are the easiest way to accumulate rxp since they provide double rxp to begin with, are at a higher level which increases the base significantly compared to pre-20 and it's much easier to find groups running at higher skulls compared to heroic.
In the early days of reaper finding R10 groups was really hard and not worth the wait so I would just solo or shortman R4 with my alts. If you run R2 up to 12, then R4 solo the rest of the way you can get to 75 reaper points reasonably fast. If you instead find a group to run R10s with at level 30 you will cut whatever that time is in half, however, that is only true if it's easy to find a group and they are running content you need first time bonuses on. If this isn't an option you can probably accumulate reaper points just as fast soloing or short-manning R4.
Reaper no longer has a lock-out preventing re-entry, so having a dual box isn't as beneficial as it used to be, but for soloing or short-manning reaper I really like having at least one alt - preferably a buffing bard which boosts skill checks and gives you really good healing with sustaining song - and sustaining song isn't degraded by the overall reaper healing debuff since it's granted by a different character. You can also raise yourself if you die which was more important when re-entry wasn't possible. It took me several years to get the content on my main alt account and I am still working on filling in the final pieces of a few accounts, but it would go much quicker if you are willing to throw money at an alt account.