How to find the right indexes for several joined tables - mysql

Question

I have the following query. the main tables are company_reports and project_con_messages with 770,000 and 1,040,000 records respectively.
When I run this query it takes about 20 seconds and when I remove the last table company_par_user_settings it takes less than 0.5 seconds.
company_par_user_settings (short name: pus) has about 200,000 records and is meant to show user settings for each company_partner. on company_par_user_settings table we have composite unique index key on partner_id and user_id fields. I also removed the index and replaced with simple indexes on partner_id and user_id but at the end, it didn't make any big difference in running time.
SELECT *
FROM company_reports rep
LEFT JOIN system_users usr ON rep.user_id=usr.id
LEFT JOIN company_rep_subjects sbj ON rep.subject_id=sbj.id
INNER JOIN company_partners par ON rep.partner_id=par.id
LEFT JOIN project_con_messages mes ON rep.message_id=mes.id
LEFT JOIN company_par_user_settings pus ON par.id=pus.partner_id AND 1=pus.user_id
WHERE 1=1
ORDER BY rep.id DESC
LIMIT 0,50
Here below I added the explain on the above query:
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
| 1 | SIMPLE | rep | NULL | ALL | partner_id | NULL | NULL | NULL | 772236 | 100.00 | Using temporary; Using filesort |
| 1 | SIMPLE | par | NULL | eq_ref | PRIMARY | PRIMARY | 3 | portal_ebrahim.rep.partner_id | 1 | 100.00 | NULL |
| 1 | SIMPLE | usr | NULL | eq_ref | PRIMARY | PRIMARY | 2 | portal_ebrahim.rep.user_id | 1 | 100.00 | Using where |
| 1 | SIMPLE | sbj | NULL | eq_ref | PRIMARY | PRIMARY | 2 | portal_ebrahim.rep.subject_id | 1 | 100.00 | NULL |
| 1 | SIMPLE | mes | NULL | eq_ref | PRIMARY | PRIMARY | 4 | portal_ebrahim.rep.message_id | 1 | 100.00 | NULL |
| 1 | SIMPLE | pus | NULL | ALL | NULL | NULL | NULL | NULL | 191643 | 100.00 | Using where; Using join buffer (Block Nested Loop) |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
I appreciate if someone help me on the right indexes or any other solutions that make the query run faster.
Edit:
Here is the show table for company_par_user_settings
CREATE TABLE `company_par_user_settings` (
`id` mediumint(9) NOT NULL AUTO_INCREMENT,
`partner_id` mediumint(8) unsigned NOT NULL,
`user_id` smallint(5) unsigned NOT NULL,
`access` tinyint(1) unsigned NOT NULL DEFAULT '0' COMMENT '0-Not specified',
`access_category` tinyint(1) unsigned NOT NULL DEFAULT '0',
`notify` tinyint(1) unsigned NOT NULL DEFAULT '0',
`stars` tinyint(1) unsigned NOT NULL DEFAULT '0',
PRIMARY KEY (`id`),
UNIQUE KEY `partner_id` (`partner_id`,`user_id`),
KEY `stars` (`stars`)
) ENGINE=MyISAM AUTO_INCREMENT=198729 DEFAULT CHARSET=utf8 COLLATE=utf8_persian_ci