I’m not as optimistic as you are that Hollister can take over the role of Allen’s strong blocking TE. I think it’s no more than 50/50 (at best) that Hollister’s even on the team next year.
During his injury-free 2017 rookie season, Hollister became a fairly high-value special-teamer. As a receiver, his route-running seemed awkward and slow, and his hads weren't great. As a blocker, any lack of upper-body strength never really came into play since his problem was that he seemed to consistently misunderstand the line calls and his blocking assignments.
During the 2018 pre-season, Hollister's route running seemed much smoother and crisper and his hands were significantly better (also evident in WK-2 @JAX). I was only able to catch a few plays in which Hollister was blocking. He seemed much more confident in his assignments and easily had adequate strength and technique, although he's certainly not anywhere near Gronk, Allen or Develin.
Injury-wise in 2018, he missed week-1 with a hammy, and then played on it in week-2 (3 catches on 3 tgts for 35 yds, +10 ST snaps). --- He was off the injury report in wk-3 until he suffered his "chest injury" near the end of Tuesday's practice. --- He missed weeks 3 & 4 with the chest issue (which I consider to be a one-off, unlikely to become a chronic, recurring thing), but played through it in week-5 (IND). He played only 4 offensive snaps against the Colts without recording a stat, but also played 19 ST snaps. --- He was off the injury report in week-6 (KCY), played only 3 snaps on offense (no stats), but 16 ST snaps.
Now, he's missing week-7 with a recurrence of the hammy issue, which may, or may not, become a chronic thing and affect his play, and playing time, all season. Until (unless) Hollister gets past this hammy issue, it's going to be impossible for me to predict any kind of trajectory for him. My current evaluation is that, when healthy, he's shown enough development to be a decent #3TE/special-teamer (a few ticks better than James O'Shaughnessy). The only projection I'd have beyond that is that he seems unlikely to become an Allen-level blocker anytime soon.