There's actually pretty good information on the state of public access Unix systems in 80's and 90's in the form the "nixpub" list that was distributed on Usenet and other places. The earliest version I could find is from November 1987 and listed 35 sites:
11/22 206-863-0453 * Sumner WA 12 24
Tandy 6000 - XENIX * Micro Magic BBS Fee $30/yr $5 setup shell access
multiple lines 863-0454/5/6
11/22 618-277-6417 herky ? IL 3/12/24 24
386 - SCO Xenix - no fee 60 min. XBBS
11/21 714-894-2246 stanton Irvine CA 12/24 24
286 - SCO Xenix - donation requested, XBBS, limit 240 min, XBBS
11/04 314-947-0895 slacbbs St. Louis MO 3/12 24
286 - SCO XENIX - no fee 60 min. - XBBS - LOGIN: bbs, shell access to
qualified REGULAR users
11/02 215-275-2429 prapc-1 Norristown PA 12 24
286 - Coherent sys7, no fee, 60 min, UNaXcess BBS, shell access, bbs
login : fhbbs (Far Horizons BBS)
11/01 212-420-0527 magpie NYC NY 3/12/24 24
? - UNIX SYSV - 2, Magpie BBS, no fee, Authors: Magpie/UNIX,/MSDOS
10/09 812-334-8453 cguild Bloomington IN 12 24
286 - SCO-XENIX V2.2.1, xbbs, also 812-334-8465
10/09 713-334-1204 nuchat Houston TX 3/12/24 24
286 - Mport USENET, mail, shell access available 120 meg
10/06 714-662-7450 turnkey Southern CA 12/24 24
286 - Xenix SYSV, XBBS
10/04 312-833-8126 vpnet Villa Park IL 3/12 24
3B1 UNIX Sys ?, no fee, shell access, AKCS/ERACS BBS
10/30 212-675-7059 marob NYC NY 12/24 24
286 SCO-XENIX 2.2, XBBS
10/31 814-333-6728 sir-alan Meadville PA 3/12/24 24
Tandy XENIX/68000 03.01.02, Allegheny College, UNaXcess BBS, anon ftp
09/24 313-623-6309 nucleus Clarkston MI 12/24 24
286 - UNIX SYS V, donation requested, AKCS/ERACS BBS
09/03 216-781-6201 ncoast Cleveland OH 3/12/24 24
PLEXSUS, no fee, donation requested -> $2.00/hr prime, $1.00/hr non-prime
08/31 206-367-3837 eskimo Seattle WA 3/12 24
Tandy-6000 XENIX, 8 lines, fee $1/mo, 1st 2 weeks free
10/18 217-529-3223 pallas Springfield IL 3/12/24 24
Convrgnt Minifrme, multiple lines, 200 meg Minnie bbs $25 donation
08/30 312-283-0559 chinet Chicago IL 3/12/24 24
3b2/300 - SYS V 3.1, multiple lines, Picospan BBS, fee $50/yr for usenet
access and other than public lines
BBS DOWN 403-295-2541 xenlink Calgary AB 3/12/24 24
286 - SCO -XENIX, no fee, Term BBS, shell access, login bbs
10/28 408-725-0561 portal Cupertino CA 3/12 24
Networked Suns (SunOS), multiple lines (some at 2400 bps), Telenet access,
fees: $10/month + Telenet charges (if used) @ various rates/times
conferencing, multi user chats, usenet, no shell access
08/29 415-332-6106 well Sausalito CA 3/12 24
VAX 750 - BSD 4.2, multiple lines, Telenet access, Picospan bbs
fees: $8/month, $3/hour direct, Telenet $20/$4 hour (peak/off peak)
08/28 214-824-7881 killer Dallas TX 3/12/24 24
3b2/400 - UNIX, no fee, various time limits, 4 lines 860 meg online
08/28 312-566-8909 ddsw1 Mundelein IL 3/12 24
286 - Mport guest usr 1 hr daily, fee extends use, ERACS/UX bbs
2400 bps for contributors($) on 312-566-8911/12 Authors of ERACS/UX bbs
10/23 312-272-5912 igloo ?? IL 12 unknown
PC7300 - UNIX limits unstated PicoSpan conference system
08/23 201-752-2820 unirot ?? NJ 12 unknown
Heurikon - Unisoft SYS V, fee status unknown AKA Soup Kitchen
08/23 714-635-2863 dhw68k Anaheim CA 3/12/24 24
Unistride 2.1, fee status unknown, avoid 0200-0700 hrs local time
08/23 212-879-9031 dasys1 NYC NY 12 unknown
Unistride - SYS V, multiple lines, fee $5/mo AKA Big Electric Cat
08/22 714-842-5851 conexch Santa Anna CA 3/12/24 24
XENIX 714-842-6348 (bbs) - 3/12 various limits fee $25/quarter XBBS
10/30 714-828-0288 alphacm Southern CA 12/24 24
286 - SCO-XENIX no fee, 60 minute per login, 4 lines, XBBS
08/02 301-540-3656-9 netsys Germantown MD 12 unknown
ALTOS 986(2) - Xenix, networked 240 meg, fee $5/mo
10/06 619-444-7006 pnet01 El Cajon CA 3/12/24 24
BSD Unix, 3 lines, contributions requested, login: pnet id: new
some USENET, net email, multi-thread conferencing. Home of
P-Net software, mail to crash!bblue or pnet01!bblue for info.
07/22 213-376-5714 pnet02 Redondo Bch CA 3/12/24 24
XENIX (also 213-374-7404) no fee, 90 min limit, login: pnet id: new
some USENET, net-work e-mail, multi-threaded conferencing
DOWN 213-459-7231 stb Sta Monica CA 3/12/24 24
Tandy 16 no limits, no fee, Serial Tree bbs (home made), shell access
10/31 305-584-4440 pinn Ft. Laud. FL 3/12/24 24
IBM AT - Microport SYS V, multiple lines, fee $12/yr MAGIC BBS
07/12 303-632-4111 chariot Colo Sprgs CO 3/12 24
Convrgnt Minifrme - SYS V, multiple lines, fee $12/mo Picospan
07/11 313-994-6333 m-net Ann Arbor MI 3/12 24
Altos 68020 - SYS III, limits unstated, fee for extended service
Picospan conference system, multiple lines, 160 meg
As you can see most of these sites ran some flavour of Xenix. Very few seem to have been using "expensive servers". Most are running on PCs, a few are running on workstations like the AT&T 3B2 that were obsolete at the time.
If you want to see more, a user by the name of "C.McCabe" has uploaded a number of old nixpub posts to Github.
Despite your claim that SDF Public Access UNIX was the most influential system, it was a late comer to the world of public access Unix. By the time SDF switched to Unix in 1990 the nixpub list had grown to over 100 entries. A much better candidate for most influential would be the Cleveland Free-Net which spawned a large number of other Free-nets in the 90's, some of which are still operating, like the National Capital Freenet. (Although I'm not sure if Cleveland Free-Net ever offered Unix shell access like the other Free-nets, it did however run on a AT&T 3B2/400 in it's early days.)
Other possible contenders for "big" early public Unix systems are Nyx and PANIX both predating SDF's switch to Unix.